Conditional handover management

ABSTRACT

A user device (UE) receives, from a source base station while the user device is operating in a source cell of the source base station, a conditional handover configuration providing information for user device operation within a candidate target cell of a candidate base station, and a corresponding condition for handing over to the candidate target cell of the candidate base station ( 1202 ). In some implementations, prior to determining that the condition is satisfied, the UE receives, from the source base station, a handover command message for immediate handover to a target base station ( 1204 ), and after receiving the handover command message, releases the conditional handover configuration ( 1206 ) and performs an immediate handover to the target base station ( 1208 ).

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Provisional PatentApplication No. 62/910,448, filed Oct. 3, 2019, the disclosure of whichis hereby incorporated herein by reference in its entirety for allpurposes.

FIELD OF THE DISCLOSURE

This disclosure relates generally to wireless communications and, moreparticularly, to conditional handover procedures.

BACKGROUND

This background description is provided for the purpose of generallypresenting the context of the disclosure. Work of the presently namedinventors, to the extent it is described in this background section, aswell as aspects of the description that may not otherwise qualify asprior art at the time of filing, are neither expressly nor impliedlyadmitted as prior art against the present disclosure.

In telecommunication systems, the Packet Data Convergence Protocol(PDCP) sublayer of the radio protocol stack provides services such astransfer of user-plane data, ciphering, integrity protection, etc. Forexample, the PDCP layer defined for the Evolved Universal TerrestrialRadio Access (EUTRA) radio interface (see 3GPP TS 36.323) and New Radio(NR) (see TS 38.323) provides sequencing of protocol data units (PDUs)in the uplink direction (from a user device, also known as a userequipment (UE), to a base station) as well as in the downlink direction(from the base station to the UE). Further, the PDCP sublayer providessignaling radio bearers (SRBs) and data radio bearers (DRBs) to theRadio Resource Control (RRC) sublayer. Generally speaking, the UE and abase station can use SRBs to exchange RRC messages as well as non-accessstratum (NAS) messages, and can use DRBs to transport data on a userplane.

UEs can use several types of SRBs and DRBs when communicating with abase station. So-called SRB1 resources carry RRC messages, which in somecases include NAS messages, over the dedicated control channel (DCCH).SRB2 resources support RRC messages that include logged measurementinformation or NAS messages, also over the DCCH but with lower prioritythan SRB1 resources. More generally, SRB1 and SRB2 resources allow theUE and the base station to exchange RRC messages.

UEs can perform handover procedures to switch from one cell to another.The UE may handover from a cell of a first base station to a cell of asecond base station, or from a cell of a first distributed unit (DU) ofa base station to a cell of a second DU of the same base station,depending on the scenario. 3GPP specifications 36.300 v15.6.0 and 38.300v15.6.0 describe a handover procedure that includes several steps (RRCsignaling and preparation) between radio access network (RAN) nodes,which cause latency in the handover procedure and therefore increase therisk of handover failure. This procedure, which does not involveconditions that are checked at the UE, can be referred to as an“immediate” handover procedure. The immediate handover proceduregenerally includes several steps (RRC signaling and preparation) betweenRAN nodes and the UE, which cause latency in the handover procedure andtherefore increase the risk of handover failure.

More recently, “conditional” handover procedures have been considered.Unlike the immediate handover procedure discussed above, the conditionalhandover procedure does not perform the handover until the UE determinesthat a condition is satisfied. To configure a conditional handoverprocedure, the RAN provides the condition to the UE, along with aconditional handover configuration (e.g., a set of random-accesspreambles, etc.) that will enable the UE to communicate with theappropriate base station, or via the appropriate cell, when thecondition is satisfied. For a conditional handover from a source basestation to a candidate base station, for example, the RAN provides theUE with a condition to be satisfied before the UE can handover to thecandidate base station, and a conditional handover configuration thatenables the UE to communicate with that candidate base station after thecondition has been satisfied.

However, opportunities exist for improving the manner in which the RANnodes manage the configurations and conditions that are associated withconditional handovers. Under recently proposed systems, for example, asource base station in the RAN might send the UE a condition that doesnot account for the state of other portions of the RAN (e.g., acondition that, if met, causes the UE to hand over to a candidate basestation that is heavily congested relative to the source base station).This, and other issues relating to conditional handover management, canresult in significant network inefficiencies (e.g., lower networkcapacity or average data rates, more radio link failures, etc.).

SUMMARY

Base stations and UEs of this disclosure implement techniques thatenable a UE to conditionally handover from a source base station or cellto a candidate base station or cell in a manner that avoids or reducesnetwork inefficiencies. In some of these techniques, for example, thesource base station provides the handover condition(s) to the candidatebase station, and the candidate base station optionally modifies (e.g.,fully or partially replaces) the condition(s) based on networkcongestion at the candidate base station and/or other suitable factors.The candidate base station may then send the source base station one ormore conditional handover configurations that enables the UE tocommunicate with the candidate base station (should the handover occur),along with the unmodified, modified and/or replaced condition(s). Insome implementations and/or scenarios, the source base station and/orthe candidate base station can modify or replace one or more conditionalhandover configurations and/or conditions that the source base stationpreviously sent to the UE. As used herein, the term “condition” mayrefer to a single, detectable state or event (e.g., a particular signalquality metric exceeding a threshold), or to a logical combination ofsuch states or events (e.g., “Condition A and Condition B,” or“(Condition A or Condition B) and Condition C”, etc.). It is understood,therefore, that the terms “condition,” “conditions” and “condition(s)”may be used interchangeably in the context of a conditional handoverprocedure for a single candidate base station. Moreover, the term“condition” may be used herein to refer to the condition in the abstract(e.g., signal quality being in a particular state), or to refer to acondition configuration (e.g., a digital representation/expression ofthe condition that can be transmitted and stored, etc.). By contrast, asused herein, the term “conditional handover configuration” refers to oneor more configurations associated with a condition that is to besatisfied before the UE can communicate with a candidate base station,or via a candidate target cell, using that configuration. Theconfigurations can include a radio bearer configuration (e.g.,RadioBearerConfig IE, DRB-ToAddMod IE or DRB-ToRelease IE), a physicallayer configuration, a MAC configuration, an RLC configuration, a PDCPconfiguration, an SDAP configuration, or any suitable configurationincluded in a CellGroupConfig IE or in a RadioResourceConfigDedicatedIE.

One example implementation of these techniques is a method in a userdevice. The method includes receiving, by processing hardware of theuser device and from a source base station while the user device isoperating in a source cell of the source base station, a conditionalhandover configuration providing information for user device operationwithin a candidate target cell of a candidate base station, and acorresponding condition for handing over to the candidate target cell ofthe candidate base station. The method also includes, prior todetermining that the condition is satisfied, receiving, by theprocessing hardware and from the source base station, a handover commandmessage for immediate handover to a target base station. The method alsoincludes, after receiving the handover command message, releasing, bythe processing hardware, the conditional handover configuration, andperforming an immediate handover to the target base station.

Another example implementation of these techniques is another method ina user device. The method includes receiving, from a source base stationand while the user device is operating in a source cell of the sourcebase station, a first conditional handover configuration providinginformation for user device operation within a first candidate targetcell of a first candidate base station. The method also includesmonitoring, by processing hardware of the user device, whether a firstcondition associated with a conditional handover to the first candidatetarget cell is satisfied. The method also includes receiving, from thesource base station and while the user device is operating in the sourcecell, a second conditional handover configuration providing informationfor user device operation within a second candidate target cell coveredby a second candidate base station. The method also includes monitoring,by the processing hardware, whether a second condition associated with aconditional handover to the second candidate target cell is satisfied.The method also includes determining, by the processing hardware andwhile monitoring whether the first and second conditions are satisfied,that the first condition is satisfied. The method also includes, inresponse to determining that the first condition is satisfied,performing a handover to the first candidate target cell, and releasing,by the processing hardware, the second conditional handoverconfiguration.

Another example implementation of these techniques is a method in a basestation. The method includes sending, by processing hardware of the basestation and to a user device while the user device is operating in asource cell of the base station, a conditional handover configurationproviding information for user device operation within a candidatetarget cell of a candidate base station, and a corresponding conditionfor handing over to the candidate target cell of the candidate basestation. The method also includes causing, by the processing hardware,the user device to release the conditional handover configuration andperform an immediate handover to a target base station prior to the userdevice determining that the condition is satisfied, at least by sending,to the user device, a handover command message for immediate handover tothe target base station.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are block diagrams of example systems in which a radioaccess network (RAN) and a user device (UE) can implement techniques ofthis disclosure for managing conditional handover procedures;

FIG. 1C is a block diagram of an example distributed base station thatcan operate in the system of FIG. 1A or FIG. 1B;

FIG. 2 is a block diagram of an example protocol stack, according towhich the UE of FIG. 1A or 1B may communicate with the base stations ofFIG. 1A or 1B, respectively;

FIG. 3 is an example message diagram corresponding to a scenario and/orimplementation in which the RAN provides an RRC container including aconditional handover configuration and corresponding condition(s) to theUE for a conditional handover procedure;

FIG. 4 is an example message diagram corresponding to a scenario and/orimplementation in which base stations of the RAN exchange condition(s)corresponding to a conditional handover configuration prior to providingthe configuration and the condition(s) to the UE for a conditionalhandover procedure;

FIG. 5 is an example message diagram corresponding to a scenario and/orimplementation in which the source base station generates a new RRCreconfiguration, including a conditional handover configuration from thecandidate base station and corresponding condition(s), and transmits thenew RRC reconfiguration to the UE for a conditional handover procedure;

FIG. 6 is an example message diagram corresponding to a scenario and/orimplementation in which an immediate handover occurs while the UE isconfigured for a conditional handover;

FIG. 7 is an example message diagram corresponding to a scenario and/orimplementation in which the UE manages a current set of conditionalhandover configurations when a condition is satisfied for a conditionalhandover;

FIG. 8 is an example message diagram corresponding to a scenario and/orimplementation in which the RAN reconfigures (e.g., replaces orotherwise modifies) a conditional handover configuration or conditionconfiguration of the UE;

FIG. 9 is an example message diagram corresponding to a scenario and/orimplementation in which the RAN reconfigures a condition configurationof the UE;

FIG. 10 is an example message diagram corresponding to a scenario and/orimplementation in which the RAN reconfigures a conditional handoverconfiguration of the UE;

FIG. 11 is a flow diagram depicting an example method, implemented in asource base station, of facilitating a conditional handover;

FIG. 12 is a flow diagram depicting an example method, implemented in aUE, of performing an immediate handover during a conditional handoverprocedure;

FIG. 13 is a flow diagram depicting an example method, implemented in aUE, of managing conditional handover configurations when handing over toa particular candidate base station;

FIG. 14 is a flow diagram depicting an example method, implemented in asource base station, of performing an immediate handover preparationprocedure or a conditional handover preparation procedure for a UE;

FIG. 15 is a flow diagram depicting an example method, implemented in asource base station, to reconfigure or release a conditional handoverconfiguration stored at a UE;

FIG. 16 is a flow diagram depicting an example method, implemented in aUE, for performing an immediate or conditional handover when acommunication failure occurs;

FIG. 17 is a flow diagram depicting an example method, implemented in aUE, for managing conditional handover configurations when performingconditional handover;

FIG. 18 is a flow diagram depicting an example method, implemented in asource base station, for providing conditions for a conditional handoverto a base station associated with a candidate target cell;

FIG. 19 is a flow diagram depicting an example method, implemented in acandidate base station, for modifying conditions for a conditionalhandover received from a source base station; and

FIG. 20 is a flow diagram depicting an example method, implemented in aRAN, for modifying a conditional handover configuration that waspreviously sent to the UE.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts an example wireless communication system 100 that canimplement techniques of this disclosure for managing conditionalhandover configurations and/or conditions. The wireless communicationsystem 100 includes a UE 102, as well as base stations 104A, 106A thatare connected to a core network (CN) 110. The wireless communicationsystem 100 can enhance connection robustness or reliability among the UE102 and the base stations 104A, 106A to enable handover of the UE 102between coverage of the base stations 104A, 106A. The base stations104A, 106A can be any suitable type, or types, of base stations, such asan evolved node B (eNB), a next-generation eNB (ng-eNB), or a 5G Node B(gNB), for example.

The base station 104A supports a cell 124A, and the base station 106Asupports a cell 126A. The cell 124A partially overlaps with cell 126A,such that the UE 102 can be in range to communicate with base station104A while simultaneously being in range to communicate with basestation 106A (or in range to detect or measure the signal from both basestations 104A and 106A, etc.). The overlap may make it possible for theUE 102 to handover between cells (e.g., from cell 124A to cell 126A)before the UE 102 experiences radio link failure, for example. Inimplementations and scenarios where the UE 102 is connected with thebase station 104A, the base station 104A operates as a source basestation (S-BS), and the base station 106A operates as a candidate basestation (C-BS). Although various scenarios are described below in whichthe base station 104A operates as an S-BS and the base station 106Aoperates as a C-BS, any of the base stations 104A, 106A generally canoperate as an S-BS or C-BS in different scenarios. Thus, in someimplementations, the base stations 104A, 106A can implement similar setsof functions and each support S-BS and C-BS operations.

In operation, the UE 102 can use a radio bearer (e.g., a DRB or an SRB)that at different times can terminate at difference base stations (e.g.,the base station 104A or the base station 106A). The UE 102 can applyone or more security keys when communicating on the radio bearer, in theuplink (from the UE 102 to a base station) and/or downlink (from a basestation to the UE 102) direction.

The base station 104A includes processing hardware 130, which mayinclude one or more general-purpose processors (e.g., central processingunits (CPUs)) and a computer-readable memory storing machine-readableinstructions executable on the general-purpose processor(s), and/orspecial-purpose processing units. The processing hardware 130 in theexample implementation of FIG. 1A includes a base station RRC controller132 that is configured to manage or control RRC procedures and RRCconfigurations. For example, the base station RRC controller 132 may beconfigured to support RRC messaging associated with immediate andconditional handover procedures, and/or to support the necessaryoperations when the base station 104A operates as an S-BS, as discussedbelow. The base station RRC controller 132 may also be configured to, insome scenarios, operate instead as a C-B S. Moreover, in someimplementations and/or scenarios, the base station RRC controller 132may be responsible for reconfiguring (for the UE 102 and a number ofother UEs not shown in FIG. 1A) current sets of conditional handoverconfigurations and conditions (i.e., condition configurations) inaccordance with various techniques discussed below.

The base station 106A includes processing hardware 140, which mayinclude one or more general-purpose processors (e.g., CPUs) and acomputer-readable memory storing machine-readable instructionsexecutable on the general-purpose processor(s), and/or special-purposeprocessing units. The processing hardware 140 in the exampleimplementation of FIG. 1A includes a base station RRC controller 142that is configured to manage or control RRC procedures and RRCconfigurations. For example, the base station RRC controller 142 may beconfigured to support RRC messaging associated with immediate andconditional handover procedures, and/or to support the necessaryoperations when the base station 106A operates as a C-BS, as discussedbelow. The base station RRC controller 142 may also be configured to, insome scenarios, operate instead as an S-BS. Moreover, in someimplementations and/or scenarios, the base station RRC controller 142may be responsible for reconfiguring (for the UE 102 and a number ofother UEs not shown in FIG. 1A) current sets of conditional handoverconfigurations and conditions in accordance with various techniquesdiscussed below. In some implementations, the RRC controller 142 issimilar to the RRC controller 132 or, more generally, the processinghardware 140 may be similar to the processing hardware 130.

The UE 102 includes processing hardware 150, which may include one ormore general-purpose processors (e.g., CPUs) and a computer-readablememory storing machine-readable instructions executable on thegeneral-purpose processor(s), and/or special-purpose processing units.The processing hardware 150 in the example implementation of FIG. 1Aincludes a UE RRC controller 152 that is configured to manage or controlRRC procedures and RRC configurations. For example, the UE RRCcontroller 152 may be configured to support RRC messaging associatedwith immediate and conditional handover procedures, and may also beresponsible for reconfiguring a current set of conditional handoverconfigurations and conditions for the UE 102 (e.g., releasing ormodifying conditional handover configurations or conditions as needed)in accordance with any of the techniques discussed below.

The CN 110 may be an evolved packet core (EPC) 111 or a fifth-generationcore (5GC) 160, both of which are depicted in FIG. 1A. Each of the basestations 104A, 106A may be an eNB supporting an S1 interface forcommunicating with the EPC 111, an ng-eNB supporting an NG interface forcommunicating with the 5GC 160, or as a base station that supports theNR radio interface as well as an NG interface for communicating with the5GC 160. To directly exchange messages with each other during thevarious scenarios discussed below, the base stations 104A, 106A maysupport an X2 or Xn interface.

Among other components, the EPC 111 can include a Serving Gateway (S-GW)112 and a Mobility Management Entity (MME) 114. The S-GW 112 isgenerally configured to transfer user-plane packets related to audiocalls, video calls, Internet traffic, etc., and the MME 114 is generallyconfigured to manage authentication, registration, paging, and otherrelated functions. The 5GC 160 includes a User Plane Function (UPF) 162and an Access and Mobility Management Function (AMF) 164, and/or aSession Management Function (SMF) 166. The UPF 162 is generallyconfigured to transfer user-plane packets related to audio calls, videocalls, Internet traffic, etc., the AMF 164 is generally configured tomanage authentication, registration, paging, and other relatedfunctions, and the SMF 166 is generally configured to manage PDUsessions.

Generally, the wireless communication system 100 may include anysuitable number of base stations supporting NR cells and/or EUTRA cells.More particularly, the EPC 111 or the 5GC 160 can be connected to anysuitable number of base stations supporting NR cells and/or EUTRA cells.For example, additional base stations are considered in immediate andconditional handover as discussed below with reference to FIGS. 1A and1B. Although the examples below refer specifically to specific CN types(EPC, 5GC) and RAT types (5G NR and EUTRA), in general the techniques ofthis disclosure also can apply to other suitable radio access and/orcore network technologies, such as a sixth generation (6G) radio accessand/or 6G core network, for example.

Example operation of various handover-related procedures that may beimplemented by the wireless communication system 100 will now bedescribed.

In some implementations, the wireless communication system 100 supportsimmediate handovers between cells. In one scenario, for example, the UE102 initially connects to the base station 104A, and the base station104A later performs preparation for an immediate handover with the basestation 106A via an interface (e.g., X2 or Xn). In this scenario, thebase stations 104A and 106A operate as a source base station and atarget base station, respectively. In the handover preparation, thesource base station 104A may send a Handover Request message to thetarget base station 106A. In response, the target base station 106A mayinclude an immediate handover command message in a Handover RequestAcknowledge message, and send the Handover Request Acknowledge messageto the source base station 104A. The source base station 104A may thentransmit the immediate handover command message to the UE 102 inresponse to receiving the Handover Request Acknowledge message.

Upon receiving the immediate handover command message, the UE 102immediately reacts to the message by attempting to connect to the targetbase station 106A. To connect to the target base station 106A, the UE102 may perform a random access procedure with the target base station106A, and then (after gaining access to a control channel) transmit aHandover Complete message to the target base station 106A via a cell ofthe target base station 106A (i.e., in response to the immediatehandover command message).

In some implementations, the wireless communication system 100 alsosupports conditional handovers. In one implementation and scenario, forexample, the UE 102 initially connects to the base station 104A, and thebase station 104A later performs a first conditional handoverpreparation procedure with the base station 106A via an interface (e.g.,X2 or Xn) to prepare for a potential handover of the UE 102 to the basestation 106A. In this scenario, the base stations 104A and 106A operateas a source base station (S-BS) and a candidate base station (C-BS),respectively. In the first conditional handover preparation procedure,the S-BS 104A sends a Handover Request message to the C-BS 106A. Inresponse, the C-BS 106A includes a first conditional handover commandmessage in a Handover Request Acknowledge message, and sends theHandover Request Acknowledge message to the S-B S 104A. The S-B S 104Athen transmits the first conditional handover command message to the UE102, in response to receiving the Handover Request Acknowledge message.

Upon receiving the first conditional handover command message, the UE102 does not immediately react to the first conditional handover commandmessage by attempting to connect to the C-BS 106A. Instead, the UE 102connects to the C-BS 106A according to the first conditional handovercommand message only if the UE 102 determines that a first condition (ora first set of conditions) is satisfied for handing over to a candidatetarget cell 126A of the C-BS 106A. The C-BS 106A provides a conditionalhandover configuration for the candidate target cell 126A (i.e., aconfiguration that the UE 102 can use to connect with the base station106A via the candidate target cell 126A) in the first conditionalhandover command message. Although the UE 102 is described as using asingle configuration to connect with the base station 106A when the UE102 determines that a single condition is satisfied for convenience, theUE 102 may determine multiple conditions and use multiple configurationsto connect with the base station 106A.

Before the first condition is met, the UE 102 has not yet connected tothe C-BS 106A. In other words, the C-BS 106A has not yet connected andserved the UE 102. In some implementations, the first condition can bethat a signal strength/quality, as measured by the UE 102 on thecandidate target cell 126A of the C-BS 106A, is “good” enough. Forexample, the first condition may be satisfied if one or more measurementresults obtained by the UE 102 (when performing measurements on thecandidate target cell 126A) are above a threshold that is configured bythe S-BS 104A, or above a pre-determined or pre-configured threshold. Inanother example, the first condition may be satisfied if one or moremeasurement results obtained by the UE 102 on the candidate target cell126A are above a threshold that is configured by the S-BS 104A, or abovean otherwise pre-determined or pre-configured threshold, and if one ormore measurement results obtained by the UE 102 on the source cell 124A(or alternatively, neighbor cell 126B) are below a threshold that isconfigured by the S-BS 104A, or above an otherwise pre-determined orpre-configured threshold. If the UE 102 determines that the firstcondition is satisfied, the C-BS 106A becomes the “target” base station106A for the UE 102, and the UE 102 attempts to connect to the targetbase station 106A. To connect to the target base station 106A, the UE102 may perform a random access procedure with the target base station106A, and then (after gaining access to a control channel) transmit afirst Handover Complete message via the candidate target cell 126A tothe target base station 106A. After the UE 102 successfully completesthe random access procedure and/or transmits the first Handover Completemessage, the target base station 106A becomes the S-BS 106A for the UE102, and the UE 102 starts communicating data with the S-BS 106A.

In some implementations and/or scenarios, conditional handovers canoccur with more than one candidate target cell supported by the C-BS106A (e.g., cell 126A and another cell of base station 106A not shown inFIG. 1A). In one such scenario, the C-BS 106A may provide a conditionalhandover configuration of an additional candidate target cell of the C-BS 106A, in addition to a conditional handover configuration of thecandidate target cell 126A, in the first conditional handover commandmessage. The UE 102 may then monitor whether a second condition is metfor the additional candidate target cell of the C-BS 106A, while alsomonitoring whether the first condition is met for the candidate targetcell 126A. The second condition can be the same as or different from thefirst condition.

In another scenario, the S-BS 104A also performs a second conditionalhandover preparation procedure with the C-BS 106A via the interface(e.g., X2 or Xn), to prepare a potential handover of the UE 102 to theC-BS 106A, in a procedure similar to that described above. In thisscenario, however, the S-BS 104A also transmits to the UE 102 a secondconditional handover command message that the S-BS 104A received fromthe C-BS 106A, for the potential handover in the second conditionalhandover preparation. The C-BS 106A may provide a conditional handoverconfiguration of an additional candidate target cell (not shown in FIG.1A) in the second conditional handover command message. The UE 102 maymonitor whether a second condition is met for the additional candidatetarget cell of the C-BS 106A. The second condition can be the same as ordifferent from the first condition.

The S-BS 104A may also perform a third conditional handover preparationprocedure with a base station 104B via an interface (e.g., X2 or Xn), toprepare a potential handover of the UE 102 to the base station 104B, ina procedure similar to that described above. The base station 104Bsupports a cell 124B that partially overlaps the cell 124A and possiblyalso the cell 126A. The base station 104B may be similar to the basestation 104A, and possibly also similar to the base station 106A, forexample. In this scenario, the S-BS 104A transmits to the UE 102 a thirdconditional handover command message, which the S-BS 104A received fromthe C-BS 104B for the potential handover in the third conditionalhandover preparation. The C-BS 104B may provide a conditional handoverconfiguration of a candidate target cell 124B in the third conditionalhandover command message. The UE 102 may monitor whether a thirdcondition is met for the candidate target cell 124B of the C-BS 104B.The third condition can be the same as or different from the firstcondition and/or the second condition. The conditional handover commandmessages above can be RRC reconfiguration messages, or may be replacedby conditional handover configurations that are information elements(IEs)).

FIG. 1B illustrates another implementation of the wireless communicationsystem 100, where the CN 110 is connected to a base station 106B inaddition to the base stations 104A, 106A, and 104B. The base station106B may be similar to the base station 106A, and possibly also similarto the base stations 104A and/or 104B. The base station 106B supports acell 126B. The cells 126B and 126A may partially overlap, such that theUE 102 can detect or measure the signal from both the base station 106Band the base station 106A while in a fixed location. In someimplementations, the base station 104A, 104B, 106A, and/or 106B supportone or more additional cells not shown in FIG. 1B. The base stations104A, 104B, 106A, and 106B may support both immediate handover andconditional handover procedures, such as those discussed above and asdiscussed in further detail below.

FIG. 1C depicts an example, distributed implementation of any one ormore of the base stations 104A, 104B, 106A, 106B. In thisimplementation, the base station 104A, 104B, 106A, or 106B includes acentralized unit (CU) 172 and one or more distributed units (DUs) 174.The CU 172 includes processing hardware, such as one or moregeneral-purpose processors (e.g., CPUs) and a computer-readable memorystoring machine-readable instructions executable on the general-purposeprocessor(s), and/or special-purpose processing units. For example, theCU 172 may include some or all of the processing hardware 130 or 140 ofFIG. 1A. The processing hardware of the CU 172 may include a basestation RRC controller (e.g., controller 142) configured to manage orcontrol one or more RRC configurations and/or RRC procedures when thebase station (e.g., base station 106A) operates as an S-BS, a C-BS, or atarget base station, for example.

Each of the DU 174s also includes processing hardware that can includeone or more general-purpose processors (e.g., CPUs) andcomputer-readable memory storing machine-readable instructionsexecutable on the one or more general-purpose processors, and/orspecial-purpose processing units. For example, the processing hardwareof the DU 174 may include a medium access control (MAC) controllerconfigured to manage or control one or more MAC operations or procedures(e.g., a random access procedure), and a radio link control (RLC)controller configured to manage or control one or more RLC operations orprocedures. The processing hardware of the DU 174 may also include aphysical layer (PHY) controller configured to manage or control one ormore physical layer operations or procedures.

FIG. 2 illustrates, in a simplified manner, an example radio protocolstack 200 according to which the UE 102 can communicate with aneNB/ng-eNB or a gNB (e.g., one or more of the base stations 104A, 104B,106A, 106B). In the example stack 200, a PHY 202A of EUTRA providestransport channels to the EUTRA MAC sublayer 204A, which in turnprovides logical channels to the EUTRA RLC sublayer 206A. The EUTRA RLCsublayer 206A in turn provides RLC channels to the EUTRA PDCP sublayer208 and, in some cases, to the NR PDCP sublayer 210. Similarly, the NRPHY 202B provides transport channels to the NR MAC sublayer 204B, whichin turn provides logical channels to the NR RLC sublayer 206B. The NRRLC sublayer 206B in turn provides RLC channels to the NR PDCP sublayer210. The UE 102, in some implementations, supports both the EUTRA andthe NR stack as shown in FIG. 2, to support handover between EUTRA andNR base stations and/or support dual connectivity (DC) over EUTRA and NRinterfaces. Further, as illustrated in FIG. 2, the UE 102 can supportlayering of NR PDCP 210 over the EUTRA RLC sublayer 206A.

The EUTRA PDCP sublayer 208 and the NR PDCP sublayer 210 receive packets(e.g., from an Internet Protocol (IP) layer, layered directly orindirectly over the PDCP layer 208 or 210) that can be referred to asservice data units (SDUs), and output packets (e.g., to the RLC layer206A or 206B) that can be referred to as protocol data units (PDUs).Except where the difference between SDUs and PDUs is relevant, thisdisclosure for simplicity refers to both SDUs and PDUs as “packets.”

On a control plane, the EUTRA PDCP sublayer 208 and the NR PDCP sublayer210 can provide SRBs to exchange RRC messages, for example. On a userplane, the EUTRA PDCP sublayer 208 and the NR PDCP sublayer 210 canprovide DRBs to support data exchange.

FIGS. 3 through 10 illustrate message sequences between the UE 102 andvarious base stations of the RAN (including base stations 104A, 106A,and/or 104B), for a number of scenarios and implementations relating tothe management of conditional handover configurations and/or conditions.

In particular, FIGS. 3 through 5 depict conditional handover scenariosin which the RAN manages (e.g., exchanges and/or packages) conditionalhandover configurations and their corresponding conditions in differentways, FIGS. 6 through 8 depict conditional handover scenarios in whichthe RAN causes the UE 102 to release a conditional handoverconfiguration and/or condition (or otherwise determine that aconditional handover configuration is no longer valid), FIG. 9 depicts aconditional handover scenario in which the RAN reconfigures (e.g.,replaces or otherwise modifies) a handover condition that was previouslysent to the UE 102, and FIG. 10 depicts a conditional handover scenarioin which the RAN reconfigures (e.g., replaces or otherwise modifies) aconditional handover configuration that was previously sent to the UE102.

Referring first to FIGS. 3 through 5, as noted above, conditionalhandover scenarios are shown in which the RAN manages (e.g., exchangesand/or packages) conditional handover configurations and theircorresponding conditions in different ways.

In FIG. 3, in a conditional handover scenario 300, the base station 104Aoperates as an S-BS for the UE 102, and the base station 106A operatesas a C-BS for the UE 102.

Initially, the UE 102 communicates 302 data (e.g., uplink (UL) data PDUsand/or downlink (DL) data PDUs) with the S-BS 104A. The S-BS 104A thenat some point determines 304 to configure a conditional handover to theC-BS 106A for the UE 102, e.g., blindly or in response to detecting asuitable event. For example, the S-BS 104A may perform the determination304 in response to the S-BS 104A receiving one or more measurementresults from the UE 102 that are above (or below) one or morepredetermined thresholds, or calculating a filtered result (from themeasurement result(s) that are above or below a predeterminedthreshold). In another example, the suitable event can be that the UE102 is moving toward the C-BS 106A. In yet another example, the suitableevent can be one or more measurement results, generated/obtained by theS-BS 104A measuring signals received from the UE 102, being above (orbelow) one or more predetermined thresholds.

After determining 304 to configure the conditional handover, the S-BS104A transmits 306 a Handover Request message to the C-BS 106A. Inresponse to the Handover Request message, the C-BS 106A includes one ormore RRC reconfigurations in a conditional handover command message(e.g., an RRC reconfiguration message, possible variations of which arediscussed below after the description of the figures), and includes theconditional handover command message in a Handover Request Acknowledgemessage. The C-BS 106A transmits 308 the Handover Request Acknowledgemessage to the S-BS 104A in response to the Handover Request message.The RRC reconfiguration(s) can include one or more conditional handoverconfigurations for a first candidate target cell (which may be called acandidate PCell (C-PCell)) of the C-B S 106A. The conditional handoverconfiguration(s) include information that would enable the UE 102 tocommunicate with the first candidate target cell (e.g., a dedicatedrandom access preamble or a set of random access preambles, physicallayer configuration, medium access control configuration, etc.), if acorresponding condition is satisfied.

In response to the Handover Request Acknowledge message, the S-BS 104Agenerates 310 an RRC container message that includes the RRCreconfiguration(s) and at least one condition corresponding to the RRCreconfiguration(s). The condition(s) may be specific to only the firstcandidate target cell, or may be specific to multiple (some or all)candidate target cells of the C-BS 106A if more than one cell is acandidate. The S-BS 104A transmits 312 the RRC container message to theUE 102. Upon receiving the RRC container message, the UE 102 can storethe RRC reconfiguration(s) and its corresponding condition(s).Optionally, the UE 102 sends 314 an RRC container response message tothe S-BS 104A in response to receiving the RRC container message. Theevents 304, 306, 308, 310, 312, and 314 are collectively referred to inFIG. 3 as the procedure 350.

In one implementation and scenario, the UE 102 determines 316 that atleast one of the condition(s) for handing over to the first candidatetarget cell of C-BS 106A is satisfied, and in response initiates 318 arandom access procedure on the first candidate target cell. The UE 102then performs 320 the random access procedure with the C-BS 106A via thefirst candidate target cell, in accordance with the conditional handoverconfiguration (e.g., random access preamble(s), etc.) of the RRCcontainer message, and transmits 322 a Handover Complete message (e.g.,an RRC reconfiguration complete message) to the C-BS 106A via thecandidate target cell, during or after the random access procedure.Because the UE 102 is already configured with RRC reconfiguration(s) andthe corresponding condition(s) by the time the UE 102 determines 316that at least one of the condition(s) is satisfied, the UE 102 iscapable of immediately performing 320 the random access procedure toquickly handover to the C-BS 106A, thereby decreasing latency in thehandover procedure.

In response to either identifying the UE 102 during the random accessprocedure or receiving the RRC reconfiguration complete message, theC-BS 106A transmits 324 a Handover Success message to the S-BS 104A, toindicate that the UE 102 is connected to the C-BS 106A. The C-BS 106Abecomes the S-BS 106A for the UE 102, and the UE 102 startscommunicating data with the S-BS 106A.

In response to receiving 324 the Handover Success message, the S-BS 104Acan optionally transmit 326 an SN Status Transfer message to the C-BS106A, and optionally forward 328 data (e.g., user plane data) receivedfrom the CN 110 (e.g., from UPF 162 or S-GW 112) intended for the UE 102to the S-BS 106A. The S-BS 106A can perform 330 a Path Switch procedurewith the CN 110, such that the CN 110 starts sending data to the S-BS106A instead of the BS 104A. The events 316, 318, 320, 322, 324, 326,328, and 330 are collectively referred to in FIG. 3 as the procedure360.

In some implementations, the S-BS 104A may specifically indicate to theC-BS 106A in the Handover Request message that the base station 106A isbeing requested for purposes of a conditional handover of the UE 102(i.e., is requested to be a C-BS for the UE 102).

In some implementations, the C-BS 106A may include/add indication(s) inthe RRC reconfiguration(s) indicating that the RRC reconfiguration(s)are associated to the C-BS 106A or the first candidate target cell. Inother implementations, the S-BS 104A may include/add the indication(s)in the RRC reconfiguration(s) after receiving the RRC reconfiguration(s)from the C-BS 106A. The indication(s) can be field name(s), specificIE(s), or condition(s) associated to the RRC reconfiguration(s), forexample.

In some implementations, the RRC container message is an RRC messagealready used for other purposes, such as an RRCConnectionReconfigurationmessage or an RRCReconfiguration message. In other implementations, theRRC container message is a new RRC message dedicated for carrying RRCreconfiguration(s) and condition(s) from the S-BS 104A to the UE 102.

In some implementations, the UE 102 uses the condition(s) to evaluatemeasurement result(s) on a candidate target cell of the C-BS 106A or aserving cell of the S-BS 104A, which may be calculated or filtered bythe UE 102. For example, if a condition includes a threshold, the UE 102can compare its measurement result(s) to the threshold to determinewhether the condition is satisfied. In another example, if a conditionincludes one or more parameters, the UE 102 can determine whether itsmeasurement result(s) and the one or more parameters satisfy anothersuitable inequality equation (e.g., according to the so-called A3 and/orA5 events defined in 3GPP TS 38.331) to determine whether the conditionis satisfied.

In another example, the condition includes a threshold for the UE 102 tocompare with its measurement result(s) on a candidate target cell of theC-BS 106A, and a configuration which controls a conditional handoverexecution probability. For example, the configuration may configure anumber, where the number is between 0 and a maximum number. If the UE102 detects that the measurement result(s) are above the threshold, theUE 102 draws/generates a random number between 0 and the maximum number.If the random number is above (or in other implementations, below) theconfigured number, the UE 102 hands over to the candidate target cell.In yet another example, the condition(s) include a first condition and asecond condition. The first condition includes a first threshold for theUE 102 to compare with its measurement result(s) on a first candidatetarget cell of the C-BS 106A, and a configuration which configures afirst priority. The second condition includes a second threshold for theUE 102 to compare with its measurement result(s) on a second candidatetarget cell of the C-BS 106A, and a configuration which configures asecond priority that is above the first priority. If the UE 102determines that the measurement result(s) on the first candidate targetcell are above both the first and second thresholds, the UE 102 handsover to the second candidate target cell because the second priority isabove the first priority.

In a scenario in which the UE 102 transmits 314 an RRC containerresponse message to the S-BS 104A in response to receiving the RRCcontainer, in one implementation, the UE 102 can set a value of atransaction identifier in the RRC container response message to a valueof a transaction identifier in the RRC container message if the RRCcontainer message and the RRC container response message included thetransaction identifiers. In some implementations, the RRC containerresponse message is an RRC message already used for other purposes, suchas an RRCConnectionReconfigurationComplete message or anRRCReconfigurationComplete message. In other implementations, the RRCcontainer response message is a new/dedicated RRC response message.

In some implementations, the RRC reconfiguration complete message isassociated to the conditional handover command message (e.g., the RRCreconfiguration message) that is included in the Handover RequestAcknowledge message. In some implementations, the RRC reconfigurationmessage includes configurations for the UE 102 to perform the randomaccess procedure with the C-BS 106A and to communicate with the C-BS106A via the first candidate target cell. In one implementation, the UE102 sets a value of a transaction identifier in the RRC reconfigurationcomplete message to a value of a transaction identifier in the RRCreconfiguration message, so that the C-BS 106A can determine that theRRC reconfiguration complete message is responsive to the RRCreconfiguration message.

In some implementations, the UE 102 transmits 322 the RRCreconfiguration complete message in a message 3 (Msg3) or in a message A(MsgA) while performing the random access procedure if the random accessprocedure is a 4-step random access procedure or a 2-step random accessprocedure, respectively. In other implementations, the UE 102 transmits322 the RRC reconfiguration complete message on a physical uplink sharedchannel (PUSCH) after performing the random access procedure.

In some implementations, the C-BS 106A can generate an identifier oridentity (ID) of the UE 102 and includes the ID in the RRCreconfiguration(s). In turn, the UE 102 includes the ID in a MAC PDU,and includes the MAC PDU in the Msg3 or the MsgA. The UE 102 transmitsthe Msg3 or MsgA to the C-BS 106A during the random access procedure, sothat the C-BS 106A can identify the UE 102. In other embodiments, the UE102 may select a dedicated random access preamble in the set of randomaccess preambles in the RRC reconfiguration(s) and transmits theselected random access preamble in the random access procedure. The C-BS106A identifies the UE 102 based on the dedicated random access preamblereceived from the UE 102 during the random access procedure. In suchembodiments, the C-BS 106A can configure the dedicated random accesspreamble in a random access configuration and includes the random accessconfiguration in the RRC reconfiguration(s).

In some implementations, the C-BS 106A consists of a CU (e.g., CU 172)and one or more DUs (e.g., DU(s) 174), as shown in FIG. 1C. The DU(s)may perform the random access procedure with the UE 102 and identify theUE 102 during the random access procedure. The CU exchanges messageswith the S-BS 104A as shown in FIG. 3 (e.g., events 306, 308, 324, 326).In some implementations, the CU generates the ID of the UE 102 andincludes the ID in the RRC reconfiguration(s). In other implementations,the CU configures a dedicated random access preamble in a random accessconfiguration and includes the random access configuration in the RRCreconfiguration(s). In yet other implementations, the CU configures theset of random access preamble in a random access configuration andincludes the random access configuration in the RRC reconfiguration(s).In some implementations, the DU generates the ID and sends the ID to theCU, which in turn includes the ID in the RRC reconfiguration(s). Inother implementations, the DU configures a dedicated random accesspreamble in a random access configuration, includes the random accessconfiguration in the RRC reconfiguration(s), and sends the RRCreconfiguration(s) to the CU. In other implementations, the DUconfigures a dedicated random access preamble in a random accessconfiguration, and sends the random access configuration to the CU,which in turn includes the random access configuration in the RRCreconfiguration(s). In yet other implementations, the UE 102 and theC-BS 106A receive the ID from the S-BS 104A or CN 110. The ID above canbe a RAN identity or a CN identity. For example, the ID can be acell-radio network temporary identifier (C-RNTI), a Temporary MobileSubscriber Identity (S-TMSI), or a 5G S-TMSI.

In some implementations, the RRC reconfiguration(s) can include one ormore cell group configuration (CellGroupConfig) information elements(IEs) that configure one or more candidate target cells. For example,each of the RRC reconfiguration(s) can include a CellGroupConfigconfiguring the first candidate target cell. In another example, the RRCreconfiguration(s) can include more than one CellGroupConfig IEsconfiguring one or more candidate target cells. The CellGroupConfig IEmay be as defined in 3GPP TS 38.331, for example. In otherimplementations, each of the RRC reconfiguration(s) is anRRCReconfiguration message as defined in 3GPP TS 38.331, for example. Insome implementations, the RRC reconfiguration(s) can include a radiobearer configuration (RadioBearerConfig).

In some implementations, the RRC reconfiguration(s) can include one ormore RRCConnectionReconfiguration-r8-IEs configuring one or morecandidate target cells. For example, each of the RRC reconfiguration(s)can include an RRCConnectionReconfiguration-r8-IEs configuring the firstcandidate target cell. In another example, the RRC reconfiguration(s)can include more than one RRCConnectionReconfiguration-r8-IEsconfiguring one or more candidate target cells. TheRRCConnectionReconfiguration-r8-IEs may be as defined in 3GPP TS 36.331,for example. In other implementations, each of the RRCreconfiguration(s) is an RRCConnectionReconfiguration message, asdefined in 3GPP TS 36.331, for example. In other implementations, theRRC reconfiguration(s) can include one or more groups of IEs configuringone or more candidate target cells. Each of the groups can include aMobilityControlInfo IE, a RadioResourceConfigDedicated IE, and aSecurityConfigHO IE, for example.

Turning now to FIG. 4, in a conditional handover scenario 400, the basestation 104A again operates as an S-BS for the UE 102, and the basestation 106A again operates as a C-BS for the UE 102. In theimplementation and scenario of FIG. 4, and unlike the implementation andscenario of FIG. 3, the S-BS 104A informs the C-BS 106A of thecondition(s) for a conditional handover before providing thecondition(s), or modified (e.g., replacement) versions thereof, to theUE 102.

Initially, the UE 102 communicates 402 data (e.g., UL data PDUs and/orDL data PDUs) with the S-BS 104. The S-BS 104A then at some pointdetermines 404 to configure a conditional handover to the C-BS 106A forthe UE 102, e.g., blindly or in response to detecting a suitable event.Event 404 may be similar to event 304 of FIG. 3, for example.

In response to the determination 404, the S-BS 104A includes thecondition(s) for the conditional handover in a Handover Request message,and transmits 406 the Handover Request message to the C-BS 106A. Thecondition(s) may otherwise be similar to the condition(s) discussedabove with reference to FIG. 3, for example.

In response to the Handover Request message, the C-BS 106A includes oneor more RRC reconfigurations and the condition(s) in a conditionalhandover command message (e.g., an RRC reconfiguration message), andincludes the conditional handover command message in a Handover RequestAcknowledge message. The C-BS 106A transmits 408 the Handover RequestAcknowledge message to the S-BS 104A in response to the Handover Requestmessage. Whereas in FIG. 3 the S-BS 104A uses an RRC container message(at events 310 and 312), in FIG. 4 the S-BS 104A transmits 412 theconditional handover command message to the UE 102 without an RRCcontainer, in response to the Handover Request Acknowledge message. Theevents 404, 406, 408, and 412 are collectively referred to in FIG. 4 asthe procedure 452.

In one implementation and scenario, the UE 102 determines that acondition for handing over to a first candidate target cell of C-BS 106Ais satisfied, and in response initiates and performs a random accessprocedure on the first candidate target cell, in a procedure 460. Theprocedure 460 may be similar to the procedure 360 of FIG. 3, forexample.

In some implementations, the C-BS 106A checks the condition(s) forconditional handover received in the Handover Request message todetermine whether to accept the Handover Request message (e.g., whetherthe condition(s) themselves are acceptable to the C-BS 106A). If theC-BS 106A determines that the condition(s) are too “loose,” the C-BS106A may send 408 a Handover Reject message or a Handover PreparationFailure message to the S-BS 104A, instead of sending 408 the HandoverRequest Acknowledge message. For example, the condition(s) may be too“loose” if a threshold that is configured by the S-BS 104A is too low,and therefore would result in one or more measurement results obtainedby the UE 102 (when performing measurements on the candidate target cell126A) frequently being above the threshold. Otherwise (i.e., if the C-BS106A determines that the condition(s) are acceptable), the C-BS 106Asends 408 the Handover Request Acknowledge message to the S-BS 104A.

In some implementations, the C-BS 106A checks the condition(s) forconditional handover received in the Handover Request message, andnegotiates the condition(s) with the S-BS 104A. The C-BS 106A may modifythe condition(s), and send 408 an interface message including themodified condition(s) to the S-BS 104A. The S-BS 104A can either rejectthe modified condition(s) by sending a Handover Cancel message to theC-BS 106A, or accept the modified condition(s) by sending another(second) Handover Request message including the modified condition(s) tothe C-BS 106A. In response, the C-BS 106A includes the modifiedcondition(s) in the conditional handover command message (e.g., an RRCreconfiguration message), and includes the conditional handover commandmessage in the Handover Request Acknowledge message.

In FIG. 5, in a conditional handover scenario 500, the base station 104Aagain operates as an S-BS for the UE 102, and the base station 106Aagain operates as a C-BS for the UE 102. In the implementation of FIG.5, unlike FIG. 4, no exchange of conditions occurs between the S-BS 104Aand C-BS 106A, and the S-BS 104A generates a new RRC reconfigurationmessage rather than forwarding the (containerized or non-containerized)RRC reconfiguration message from the C-BS 106A to the UE 102 as in FIGS.3 and 4.

Initially, the UE 102 communicates 502 data (e.g., UL data PDUs and/orDL data PDUs) with the S-BS 104. The S-BS 104A then at some pointdetermines 504 to configure a conditional handover to the C-BS 106A forthe UE 102, e.g., blindly or in response to detecting a suitable event.Event 504 may be similar to event 304 of FIG. 3, for example.

After determining 504 to configure the conditional handover, the S-BS104A transmits 506 a Handover Request message to the C-BS 106A. Inresponse, the C-BS 106A includes one or more RRC reconfigurations in aconditional handover command message (e.g., a first RRC reconfigurationmessage), includes the conditional handover command message in aHandover Request Acknowledge message, and transmits 508 the HandoverRequest Acknowledge message to the S-BS 104A. Events 504, 506 and 508may be similar to events 304, 306 and 308 of FIG. 3, for example.

In response to the Handover Request Acknowledge message, the S-BS 104Aextracts all the RRC reconfiguration(s) from the first RRCreconfiguration message and generates 511 a second RRC reconfigurationmessage including the condition(s) and the extracted RRCreconfiguration(s). The condition(s) themselves may be similar to thecondition(s) discussed above with reference to FIG. 3 or FIG. 4, forexample.

The S-BS 104A transmits 512 the second RRC reconfiguration message tothe UE 102. The events 504, 506, 508, 511, and 512 are collectivelyreferred to in FIG. 5 as the procedure 554.

In one implementation and scenario, the UE 102 determines that acondition for handing over to a first candidate target cell of C-BS 106Ais satisfied, and in response initiates and performs a random accessprocedure on the first candidate target cell, in a procedure 560. Theprocedure 560 may be similar to the procedure 360 of FIG. 3, forexample.

In some implementations, the S-BS 104A sets a transaction identifier inthe second RRC reconfiguration message to a value of a transactionidentifier in the first RRC reconfiguration message. In otherimplementations, the S-BS 104A directly includes the condition(s) thatwere in the first RRC reconfiguration message into the second RRCreconfiguration message. In yet other implementations, the S-BS 104Agenerates and directly includes the condition(s) into the second RRCreconfiguration message.

Referring to FIGS. 6 through 8, as noted above, conditional handoverscenarios are shown in which the RAN causes the UE 102 to release aconditional handover configuration and/or condition (or otherwisedetermine that a conditional handover configuration is no longer valid).

In FIG. 6, in a conditional handover scenario 600, the base station 104Aoperates as an S-BS for the UE 102, the base station 106A operates as aC-BS for the UE 102, and the base station 104B operates as a target basestation (T-BS) for the UE 102. In the scenario 600, the S-BS 104Adetermines to configure an immediate handover to the T-BS 104B for theUE 102 while the UE 102 is configured for a conditional handover.

Initially, the UE 102 communicates 602 data (e.g., UL data PDUs and/orDL data PDUs) with the S-BS 104A. The S-BS 104A then at some pointdetermines to configure a conditional handover to the C-BS 106A for theUE 102, in a procedure 650/652/654. The procedure 650/652/654 may besimilar to any one of the procedures 350, 452, or 554 of FIG. 3, 4, or5, respectively, for example.

In one implementation and scenario, prior to the UE 102 determining thata condition for handing over to a first candidate target cell of C-BS106A is satisfied, the S-BS 104A determines 605 to configure animmediate handover to a target cell of the T-BS 104B for the UE 102. TheS-BS 104A may determine 605 to configure the immediate handover inresponse to receiving measurement result(s) of the target cell from theUE 102 and determining that the measurement result(s) satisfy a certainthreshold, for example. After determining 605 to configure the immediatehandover, the S-BS 104A transmits 607 an (immediate) Handover Requestmessage to the T-BS 104B. In response to the (immediate) HandoverRequest message, the T-BS 104B includes one or more RRC reconfigurationsin a handover command message (e.g., an RRC reconfiguration message),and includes the handover command message in a Handover RequestAcknowledge message. The T-BS 104B transmits 609 the Handover RequestAcknowledge message to the S-BS 104A in response to the (immediate)Handover Request message.

In response to the Handover Request Acknowledge message, the S-BS 104Atransmits 613 the handover command message containing the RRCreconfiguration(s) to the UE 102. Upon receiving the handover commandmessage, the UE 102 immediately reacts to the handover command message,by attempting to connect to the T-BS 104B. To connect to the T-BS 104B,the UE 102 performs 620 a random access procedure via the target cell ofthe T-BS 104B, and then transmits 622 a Handover Complete message (e.g.,an RRC reconfiguration complete message) to the T-BS 104B. After the UE102 successfully completes the random access procedure and/or transmitsthe Handover Complete message, the T-BS 104B becomes the S-BS 104B forthe UE 102, and the UE 102 starts communicating data with the S-BS 104B.

In some implementations, the UE 102 transmits 622 the RRCreconfiguration complete message in a Msg3 or in a MsgA while performingthe random access procedure if the random access procedure is a 4-steprandom access procedure or a 2-step random access procedure,respectively. In other implementations, the UE 102 transmits 622 the RRCreconfiguration complete message on a PUSCH after performing the randomaccess procedure.

In some implementations, the determination 605 by the S-BS 104A that theUE 102 should perform an immediate handover procedure supersedes thepreviously initiated conditional handover procedure (e.g., renders theconditional handover procedure unnecessary). Thus, the UE 102 releases631 the conditional handover configuration after the immediate handoverto the T-BS 104B. This prevents the UE 102 from unnecessarily storingand/or accounting for the conditional handover configuration and itscorresponding condition. In such implementations, the UE 102 may beconfigured to release 631 the conditional handover configuration inresponse to receiving the handover command message at event 613, orafter successfully performing 620 the random access procedure. In someimplementations, the handover command message may include an indication(e.g., field or an IE) to release the conditional handoverconfiguration. In other implementations, the UE 102 may autonomouslyrelease 631 the conditional handover configuration regardless of whetherthe handover command message includes such an indication.

Similarly, the C-BS 106A may also unnecessarily be storing and/oraccounting for the conditional handover configuration. Accordingly, insome implementations, the S-BS 104A determines to instruct the C-BS 106Ato release the conditional handover configuration in response to theimmediate handover of the UE 102 to the T-BS 104B (e.g., in response toreceiving a Handover Success message from the T-BS 104B). In one suchimplementation, after the immediate handover of the UE 102 to the T-BS104B, the S-BS 104A sends a Handover Cancel Request message, oralternatively, a Handover Cancel message, to the C-B S 106A. In turn,the C-BS 106A releases the conditional handover configuration. The C-BS106A can optionally send a Handover Cancel Response message to the S-BS104A in response to the Handover Cancel Request message. In another suchimplementation, instead of releasing the conditional handoverconfiguration in response to the Handover Cancel Request message orHandover Cancel message, the C-BS 106A automatically releases theconditional handover configuration if the C-BS 106A does not connect tothe UE 102 within a certain time period threshold. The C-BS 106A canspecify the time period threshold, or receive the time period thresholdfrom the S-BS 104A in the Handover Request message during procedure650/652/654.

In some implementations, similar to the communications among S-BS 104A,C-BS 106A, and CN 110 discussed above with reference to FIG. 3, in FIG.6 the S-BS 104A can optionally transmit 626 an SN Status Transfermessage to the T-BS 104B, and optionally forward 628 data received fromthe CN 110 intended for the UE 102 to the T-BS 104B, after receiving 609the Handover Request Acknowledge message. Events 626, 628 may occurbefore event 620, as depicted in FIG. 6, or after event 622, forexample. The T-BS 104B can perform 630 a Path Switch procedure with theCN 110, such that the CN 110 starts sending data to the T-BS 104Binstead of the S-BS 104A.

In some implementations, in response to receiving the handover commandmessage at event 613, and to communicate data with the T-BS 104B, the UE102 derives a security key (e.g., K_(eNB) or K_(gNB)) associated to theT-BS 104B, e.g., from a security key (e.g., K_(eNB) or K_(gNB))associated to the S-BS 104A or from another security key (e.g., K_(AMF)or NH). The UE 102 derives at least one ciphering key (e.g., K_(RRCenc),K_(UPenc)) and at least one integrity key (e.g., K_(RRCint), K_(UPinc))from the security key associated to the T-BS 104B. The UE 102communicates data with the T-BS 104B using the derived at least oneciphering key (e.g., K_(RRCenc), K_(UPenc)) and/or at least oneintegrity key (e.g., K_(RRCint), K_(UPinc)).

In some implementations, instead of releasing 631 the conditionalhandover configuration after handover to the T-BS 104B, the UE 102 keepsthe conditional handover configuration received from the C-BS 106A, andalso keeps a security key (e.g., K_(eNB) or K_(gNB)) associated to theS-BS 104A. In this way, if the UE 102 detects a condition for handingover to the first candidate target cell of the C-BS 106A, the UE 102 isstill able to perform conditional handover to the C-BS 106A according tothe kept conditional handover configuration, and then communicate datawith the C-BS 106A using the security key. If the UE 102 hands over tothe C-BS 106A, the UE 102 can derive a security key associated to theC-BS 106A from the security key associated to the S-BS 104A instead ofthe security key associated to the T-BS 104B. Then, the UE 102 derivesat least one ciphering key (e.g., K_(RRCenc), K_(UPenc)) and at leastone integrity key (e.g., K_(RRCint), K_(UPinc)) from the security keyassociated to the C-BS 106A. In one implementation, the C-BS 106Areceives the same security key (e.g., K_(eNB) or K_(gNB)) as that of theUE 102 from the S-BS 104A, and uses the security key to derive the sameciphering key(s) (e.g.,K_(RRcenc), K_(UPenc)) and integrity key(s)(e.g., K_(RRCint), K_(UPinc)) as derived by the UE 102. In anotherimplementation, instead of deriving the ciphering key(s) and integritykey(s) from the security key, the C-BS 106A receives the cipheringkey(s) and integrity key(s) from the S-BS 104A, either directly (e.g.,via X2/Xn interface) or indirectly (e.g., via S1/NG interface).

In other implementations, instead of keeping the security key (e.g.,K_(eNB) or K_(gNB)) associated to the S-BS 104A, deriving the securitykey associated to the C-BS 106A from the security key associated to theS-BS 104A, and deriving the ciphering key(s) and integrity key(s) fromthe security key associated to the C-BS 106A, the UE 102 derives asecurity key (e.g., K_(eNB) or K_(gNB)) from a security configurationincluded in the conditional handover configuration received from theC-BS 104B, and subsequently derives the ciphering key(s) and integritykey(s) from the security key. The C-BS 106A can derive the sameciphering key(s) and integrity key(s) in a similar manner.

In some scenarios, the UE 102 may fail to handover to the T-BS 104B,e.g., due to failing the random access procedure with the T-BS 104B viathe target cell or expiration of a handover guarding timer (e.g., T304)for executing the immediate handover. The UE 102 may perform an RRCreestablishment procedure (e.g., RRC Connection Reestablishmentprocedure as specified in 3GPP Technical Specification 36.331 or RRCReestablishment procedure as specified in 3GPP Technical Specification38.331). In such scenarios, the UE 102 may either keep or release theconditional handover configuration received from the S-BS 104A, asdiscussed above. The UE 102 may also release any secondary cell(s)(SCell(s)) if configured. In these scenarios, the UE 102 may keep orrelease measurement configuration(s) in response to the RRCreestablishment procedure. For example, in response to the RRCreestablishment procedure, the UE 102 may keep a first measurementconfiguration and release a second measurement configuration, keep allof the measurement configuration(s), or release all of the measurementconfiguration(s).

Turning now to FIG. 7, in a conditional handover scenario 700 the basestation 104A operates as an S-BS for the UE 102, the base station 106Aoperates as a C-BS for the UE 102, and the base station 104B operates asanother C-BS for the UE 102. In the scenario 700, the UE 102 manages acurrent set of conditional handover configurations when a condition issatisfied for a conditional handover.

Initially, the UE 102 communicates 702 data (e.g., UL data PDUs and/orDL data PDUs) with the S-BS 104A. The S-BS 104A then at some pointdetermines to configure a first conditional handover, and configures theUE 102 with a first conditional handover configuration and one or morecorresponding first conditions, for a first conditional handover to theC-BS 106A, in procedure 750A/752A/754A. In one implementation andscenario, the S-BS 104A may also determine to configure the UE 102 witha second conditional handover configuration and one or morecorresponding second conditions, for a second conditional handover tothe C-BS 104B, in procedure 750B/752B/754B. Each of the procedures750A/752A/754A and 750B/752B/754B may be similar to any one of theprocedures 350, 452, or 554 of FIG. 3, 4, or 5, respectively.Accordingly, the S-BS 104A may configure the UE 102 with multipleconditional handover configurations to be used for conditional handoverto different candidate base stations.

The UE 102 then detects that the condition(s) corresponding to the firstconditional handover is/are satisfied, and hands over to the C-BS 106Avia a first candidate target cell, in procedure 760. The procedure 760may be similar to any one of the procedures 360, 460, or 560 of FIG. 3,4, or 5, respectively. Accordingly, the C-BS 106A becomes the S-BS 106Afor the UE 102, and the UE 102 starts communicating data with the S-BS106A.

In some implementations, the UE 102 determines 731A that the firstconditional handover configuration is no longer valid for conditionalhandover because the first conditional handover configuration hasalready been used by the UE 102 to handover to the S-BS 106A duringprocedure 760. Thus, if the UE 102 later on detects a failure (e.g.,radio link failure) on a communication link with the S-BS 106A andselects back to the first candidate target cell associated to the firstconditional handover configuration, the UE 102 will not hand over to theS-BS 106A using the first conditional handover configuration. In otherimplementations, rather than the UE 102 determining 731A that the firstconditional handover configuration is no longer valid, the S-BS 106A maydetermine that the first conditional handover configuration is no longervalid. Thus, if the UE 102 attempts to select back to the firstcandidate target cell and perform the random access procedure with theS-BS 106A, the S-BS 106A can identify the UE 102 during the randomaccess procedure and prevent the UE 102 from handing over to the S-BS106A using the first conditional handover configuration. In oneimplementation, the S-BS 106A can transmit a handover reject message tothe UE 102, e.g., during or after the random access procedure, toprevent the UE 102 from handing over to the S-BS 106A. In anotherimplementation, the S-BS 106A does not respond to the UE 102 afteridentifying the UE 102 during the random access procedure.

In some implementations, if the UE 102 hands over to the C-BS 106A andthereby renders the second conditional handover configuration (ifconfigured by the S-BS 104A in procedure 750B/752B/754B) unnecessary,the UE 102 releases 731B the second conditional handover configuration,similar to the manner of releasing 631 the conditional handoverconfiguration discussed above with reference to FIG. 6. Moreover, theS-BS 104A may instruct the C-BS 104B to release the second conditionalhandover configuration in response to the handover of the UE 102 to theC-BS 106A, similar to the manner in which the S-BS 104A instructs theC-BS 106A to release the conditional handover configuration in responseto the handover of the UE 102 to the T-BS 104B, as discussed above withreference to FIG. 6.

In some implementations, after the UE 102 hands over to the C-BS 106Avia the first candidate target cell in procedure 760, the UE 102communicates data with the C-BS 106A using derived ciphering key(s)and/or integrity key(s), similar to the manner in which the UE 102communicates data with the T-BS 104B, as discussed above with referenceto FIG. 6.

In some implementations, instead of releasing 731B the secondconditional handover configuration after handover to the C-BS 106A, theUE 102 keeps the second conditional handover configuration received fromthe C-BS 104B, and also keeps a security key associated to the S-BS104A, similar to the manner in which the UE 102 can keep the conditionalhandover configuration received from the C-BS 106A and the security keyassociated to the S-BS 104A, as discussed above with reference to FIG.6.

In some scenarios, the UE 102 may fail to handover to the C-BS 106A(e.g., due to failing the random access procedure with the C-BS 106A viathe first candidate target cell). In such scenarios, the UE 102 mayeither keep or release the first conditional handover configuration, andkeep the second conditional handover configuration so that the UE 102 isstill able to perform handover to the C-BS 104B using the secondconditional handover configuration.

Turning now to FIG. 8, in a conditional handover scenario 800, the basestation 104A operates as an S-BS for the UE 102, and the base station106A operates as a C-BS for the UE 102. Unlike the implementations andscenarios of FIGS. 6 and 7 in which the UE 102 decides to release aconditional handover configuration that is rendered unnecessary as aresult of the UE 102 successfully handing over to a base station (e.g.,T-BS 104B, C-BS 106A), in this implementation and scenario 800, the S-BS104A instructs the UE 102 to release a conditional handoverconfiguration to prevent the UE 102 from attempting to handover to C-BS106A.

Initially, the UE 102 communicates 802 data (e.g., UL data PDUs and/orDL data PDUs) with the S-BS 104A. The S-BS 104A then at some pointdetermines to configure a conditional handover, and configures the UE102 with at least one conditional handover configuration, and one ormore corresponding conditions, for conditional handover to the C-BS106A, in procedure 850/852/854. The procedure 850/852/854 may be similarto any one of the procedures 350, 452, or 554 of FIG. 3, 4, or 5,respectively.

In one implementation and scenario, prior to the UE 102 determining thata condition for handing over to a first candidate target cell of C-BS106A is satisfied, the S-BS 104A determines 861 to reconfigure (e.g.,replace, modify) or release the configuration(s) previously sent to theUE 102 for communication at event 802, or to add new configuration(s) tothe UE 102. For example, the S-BS 104A may determine 861 to reconfigureor release the configuration(s) if the S-BS 104A becomes congested orhas less data to transmit to the UE 102 after having previously sent theconditional handover configuration(s) to the UE 102. The S-BS 104A canassign less radio resources to the UE 102 in reconfiguredconfiguration(s) than in the previously sent configuration(s), orrelease the configuration(s), for example. In another example, the S-BS104A may determine 861 to reconfigure the configuration(s) or add thenew configuration(s) if the S-BS 104A becomes uncongested or has moredata to transmit to the UE 102 after having previously sent theconditional handover configuration(s) to the UE 102. The S-BS 104A canassign more radio resources to the UE 102 in reconfigured or newly addedconfiguration(s) than in the previously sent configuration(s), forexample. In some implementations, the configuration(s) (i.e., previouslysent configuration(s), reconfigured configuration(s), newconfiguration(s)) can include a radio bearer configuration (e.g.,RadioBearerConfig IE, DRB-ToAddMod IE or DRB-ToRelease IE). In otherimplementations, the configuration(s) can include one or moreconfigurations in a CellGroupConfig IE or in aRadioResourceConfigDedicated IE.

In some scenarios, the C-BS 106A may be unaware that the S-BS 104Areconfigured the previously sent configuration(s). In such a case, ifthe UE 102 were to determine that a condition for handing over to theC-BS 106A is satisfied, a communication error may result from the UE 102attempting to use the reconfigured configuration(s) to communicate withthe C-BS 106A. To prevent such communication errors, in response to theS-BS 104A determining 861 to reconfigure the configuration(s) or addingnew configuration(s), the S-BS 104A may determine to release 862 theconditional handover configuration previously sent to the UE 102 duringprocedure 850/852/854. In response to the determination 862, the S-BS104A transmits 865 an RRC reconfiguration message to the UE 102. The RRCreconfiguration message includes an indication to release the previouslysent conditional handover configuration. In response to receiving theRRC reconfiguration message, the UE 102 releases 867 the previously sentconditional handover configuration. By releasing 867 the previously sentconditional handover configuration, the UE 102 is prevented fromperforming the conditional handover to the C-BS 106A using thepreviously sent conditional handover configuration. In response to thedetermination 862, the S-BS 104A may optionally also transmit 864 aHandover Cancel message (or Handover Cancel Request message) to the C-BS106A, for the C-BS 106A to also release the conditional handoverconfiguration.

In some scenarios, in response to the S-BS 104A determining 861 toreconfigure the configuration(s), the S-BS 104A determines 863 torelease the condition(s) for the UE 102, in addition to or instead ofdetermining 862 to release the previously sent conditional handoverconfiguration. In response to the determination 863, the S-BS 104Atransmits 866 an RRC reconfiguration message to the UE 102. The RRCreconfiguration message indicates (e.g., instructs the UE 102 to)release of the condition(s). In response to receiving the RRCreconfiguration message, the UE 102 releases 868 the condition(s). Byreleasing 868 the condition(s), the UE 102 is prevented from determiningthat the condition(s) are satisfied, thereby preventing the UE 102 fromperforming the conditional handover to the C-BS 106A.

In response to receiving the RRC reconfiguration message at events 865and/or 866, the UE 102 transmits 869 an RRC reconfiguration completemessage to the S-BS 104A.

In some implementations, the S-BS 104A transmits 865 the RRCreconfiguration message to the UE 102, before or after transmitting 864the Handover Cancel message to the C-BS 106A. In other implementations,the S-BS 104A may transmit 865 the RRC reconfiguration message to the UE102 and transmit 864 the Handover Cancel message to the C-BS 106Asimultaneously.

In some implementations, the C-BS 106A may also release the conditionalhandover configuration. In one such implementation, C-BS 106A consistsof a CU (e.g., CU 172) and at least one DU (e.g., DU 174), as shown inFIG. 1C. In procedure 850/852/854, the CU 172 receives one or moreconditional handover configurations from the DU(s) 174. The CU 172receives the Handover Cancel message from the S-BS 104A at event 864,and in response, sends a UE Context Release Command message to each ofthe DU(s) 174. Each of the DU(s) 174 releases a particular one of theconditional handover configuration(s) in response to the UE ContextRelease Command message. Each of the DU(s) 174 may send a UE ContextRelease Complete message to the CU 172 in response to the UE ContextRelease Command message.

In FIG. 9, in a conditional handover scenario 900 the base station 104Aoperates as an S-BS for the UE 102, and the base station 106A operatesas a C-BS for the UE 102. Unlike the implementations and scenarios ofFIG. 8 in which the S-BS 104A instructs the UE 102 to release aconditional handover configuration to prevent the UE 102 from attemptingto handover to C-BS 106A, in the scenario 900, the S-BS 104A instructsthe UE 102 to reconfigure the condition corresponding to the conditionalhandover configuration.

Initially, the UE 102 communicates 902 data (e.g., UL data PDUs and/orDL data PDUs) with the S-BS 104A. The S-BS 104A then at some pointdetermines to configure a conditional handover, and configures the UE102 with at least one conditional handover configuration, and one ormore corresponding conditions, for conditional handover to the C-BS106A, in procedure 950/952/954. The procedure 950/952/954 may be similarto any one of the procedures 350, 452, or 554 of FIG. 3, 4, or 5,respectively.

In one implementation and scenario, prior to the UE 102 determining thata condition for handing over to a first candidate target cell of C-BS106A is satisfied, the S-BS 104A determines 971 to reconfigure (e.g.,replace, modify) or release the condition(s) for the UE 102. In someimplementations, the S-BS 104A determines 971 to reconfigure thecondition(s) if the S-BS 104A reconfigures or releases configuration(s)previously sent to the UE 102 for communication at event 902, or to addnew configuration(s) to the UE 102. In response to the S-BS 104Adetermining 971 to reconfigure the condition(s), the S-BS 104A transmits972 an RRC reconfiguration message including the reconfiguredcondition(s) to the UE 102. In turn, the UE 102 reconfigures 973 thecondition(s), and transmits 974 an RRC reconfiguration complete messageto the S-BS 104A in response to the RRC reconfiguration message. In someimplementations, the UE 102 may release the conditional handoverconfiguration(s) corresponding to the condition(s) previously sentduring the procedure 950/952/954 in response to receiving the RRCreconfiguration message at event 972 or in response to reconfiguring 973the condition(s).

In some implementations, the S-BS 104A determines 971 to reconfigure thecondition(s) such that the UE 102 has little or no chance of determiningthat the reconfigured condition(s) are satisfied, thereby preventing theUE 102 from attempting to connect to the C-BS 106A. In response to thedetermination 971, the S-BS 104A transmits 972 the reconfiguredcondition(s) in an RRC reconfiguration message. In response to receiving972 the RRC reconfiguration message, the UE 102 reconfigures 973 thecondition(s). In one implementation, the S-BS 104A transmits 972 the RRCreconfiguration message releasing the condition(s). In response toreceiving 972 the RRC reconfiguration message, the UE 102 releases 973the condition(s).

In other implementations, the S-BS 104A determines 971 to reconfigurethe condition(s) based on network congestion. In one suchimplementation, to prevent the UE 102 from handing over to a congestedC-BS 106A, the S-BS 104A can determine 971 to reconfigure thecondition(s) in response to receiving a notification (e.g., from theC-BS 106A, a core network or an operation and maintenance (O&M) node)that the C-BS 106A is congested. If one of the condition(s) received inprocedure 950/952/954 includes a low threshold for the UE 102 to comparewith its measurement result(s) on a candidate target cell of the C-BS106A, the reconfigured condition(s) may include a higher threshold sothat the UE 102 is less likely to detect that the reconfiguredcondition(s) is satisfied and hand over to the C-BS 106A. In anothersuch implementation, to expedite handover to C-BS 106A, e.g., due tolimited available resources or congestion at the S-BS 104A, the S-BS104A can determine 971 to reconfigure the condition(s) to include athreshold that is lower than the one included in the condition(s)received in procedure 950/952/954, so that the UE 102 is more likely todetect, or more likely to detect at an earlier time, that thereconfigured condition(s) are satisfied.

In FIG. 10, in a conditional handover scenario 1000, the base station104A operates as an S-BS for the UE 102, and the base station 106Aoperates as a C-BS for the UE 102. Unlike the implementations andscenarios of FIGS. 8 and 9 in which the S-BS 104A instructs the UE 102to release a conditional handover configuration or its correspondingcondition to prevent the UE 102 from attempting to hand over to C-BS106A, in the scenario 1000, the S-BS 104A provides a reconfiguredconditional handover configuration to UE 102 to enable the UE 102 tohandover to C-BS 106A.

Initially, the UE 102 communicates 1002 data (e.g., UL data PDUs and/orDL data PDUs) with the S-BS 104A. The S-BS 104A then at some pointdetermines to configure a conditional handover, and configures the UE102 with at least one conditional handover configuration, and one ormore corresponding conditions, for conditional handover to the C-BS106A, in procedure 1050/1052/1054. The procedure 1050/1052/1054 may besimilar to any one of the procedures 350, 452, or 554 of FIG. 3, 4, or5, respectively.

In one implementation and scenario, prior to the UE 102 determining thata condition for handing over to a first candidate target cell of C-BS106A is satisfied, the S-BS 104A determines 1061 to reconfigure (e.g.,replace or otherwise modify) the conditional handover configuration(s)previously sent to the UE 102 during procedure 1050/1052/1054. Forexample, the S-BS 104A determines 1061 to reconfigure the conditionalhandover configuration(s) in response to determining to reconfigure orrelease configuration(s) previously sent to the UE 102 for communicationat event 802, or to add new configuration(s) to the UE 102, as describedfor event 861. In response, the S-BS 104A transmits 1063 a modificationrequest message (e.g., Handover Request message, Handover ModificationRequest) to the C-B S 106A, which in turn generates a reconfiguredconditional handover configuration in response to the modificationrequest message.

In another implementation and scenario, rather than the S-BS 104Adetermining 1061 to reconfigure the conditional handover configuration,the C-BS 106A determines 1062 to reconfigure the conditional handoverconfiguration. The C-BS 106A may determine 1062 to reconfigure theconditional handover configuration if the C-BS 106A becomes congestedafter having previously sent the at least one conditional handoverconfiguration to the UE 102, for example. The C-BS 106A can assign lessradio resources to the UE 102 in the reconfigured conditional handoverconfiguration than in the previously sent conditional handoverconfiguration. In response, the C-BS 106A generates a reconfiguredconditional handover configuration. The C-BS 106A may determine 1062 toreconfigure the conditional handover configuration if the C-BS 106Abecomes uncongested after having previously sent the at least oneconditional handover configuration to the UE 102, in another example.The C-BS 106A can assign more radio resources to the UE 102 in thereconfigured conditional handover configuration than in the previouslysent conditional handover configuration. In response, the C-BS 106Agenerates a reconfigured conditional handover configuration.

In any event, after the C-BS 106A generates the reconfigured conditionalhandover configuration, the C-BS 106A includes the reconfiguredconditional handover configuration in a modification message (e.g., aHandover Request Acknowledge message, Handover Modification message, orHandover Cancel message) and transmits 1064 the modification message tothe S-BS 104A. In response, the S-BS 104A transmits 1065 thereconfigured conditional handover configuration to the UE 102, which inturn reconfigures 1066 the previously sent conditional handoverconfiguration according to the reconfigured conditional handoverconfiguration. The UE 102 may optionally transmit an RRC reconfigurationcomplete message to the S-BS 104A in response to receiving thereconfigured conditional handover configuration.

In some implementations, the S-BS 104A transmits a Handover Cancelmessage (or Handover Cancel Request message) to the C-BS 106A beforetransmitting 1063 the modification request message to the C-BS 106A. TheS-BS 104A uses the Handover Cancel Request message to request the C-BS106A to release the conditional handover configuration that waspreviously configured for the UE 102. Then, the C-BS 106A generates thereconfigured conditional handover configuration in response to themodification request message, and includes the reconfigured conditionalhandover configuration in the modification message. In response toreceiving the modification message, the S-BS 104A provides 1065 thereconfigured conditional handover configuration to the UE 102, which inturn reconfigures the previously sent conditional handover configurationwith the reconfigured conditional handover configuration.

In some implementations, the S-BS 104A determines 1061 to reconfigurethe conditional handover configuration in response to reconfiguring aradio bearer configuration (e.g., RadioBearConfig) after having alreadyprovided the radio bearer configuration to the UE 102 in the procedure1050/1052/1054. In one such implementation, if the S-BS 104A releases aradio bearer of the UE 102 after having already provided the radiobearer configuration to the UE 102, the S-BS 104A transmits 1063 theHandover Request message to the C-BS 106A that either includes areconfigured radio bearer configuration that excludes the released radiobearer, or includes an indication for the C-BS 106A to exclude thereleased radio bearer when generating the reconfigured radio bearerconfiguration. In turn, the C-BS 106A generates the reconfiguredconditional handover configuration that excludes one or moreconfigurations corresponding to the released radio bearer. In anothersuch implementation, if the S-BS 104A adds a radio bearer of the UE 102after having already provided the radio bearer configuration to the UE102, the S-BS 104A transmits 1063 a Handover Request message to the C-BS106A that either includes a reconfigured radio bearer configuration thatincludes the added radio bearer, or includes an indication for the C-BS106A to include the added radio bearer when generating the reconfiguredradio bearer configuration. In turn, the C-BS 106A generates thereconfigured conditional handover configuration that includes one ormore configurations corresponding to the added radio bearer.

In some implementations, after the UE 102 and the S-BS 104A deriveciphering key(s) and integrity key(s) from a security key to communicatedata, the S-BS 104A generates a new security key and sends 1063 the newsecurity key in a Handover Request message to the C-BS 106A. In turn,the C-BS 106A generates a reconfigured conditional handoverconfiguration, which includes a new security configuration and otherconfigurations (e.g., a physical layer configuration, a MACconfiguration, an RLC configuration, a PDCP configuration, an SDAPconfiguration, and/or a radio bearer configuration). The C-BS 106A thenincludes the reconfigured conditional handover configuration in theHandover Request Acknowledge message, and transmits 1064 the HandoverRequest Acknowledge message to the S-BS 104A.

FIG. 11 is a flow diagram depicting an example method 1100, implementedin a source base station (e.g., S-BS 104A) to facilitate a conditionalhandover.

In the method 1100, at block 1102, the source base station transmits(e.g., in any one of events 306, 406, or 506) a Handover Request messageto a candidate base station (e.g., C-BS 106A) that includes anindication to request a conditional handover configuration for aconditional handover from the candidate base station. The conditionalhandover configuration is associated with a condition to be satisfiedbefore the user device can communicate with the candidate base station,or via a candidate target cell (e.g., cell 126A) of the candidate basestation, using the conditional handover configuration. In someimplementations, the Handover Request message includes the condition.

At block 1104, the source base station receives (e.g., in any one ofevents 308, 408, 508) a Handover Request Acknowledge message from thecandidate base station that includes the conditional handoverconfiguration. In implementations in which the Handover Request messageincludes the condition, the Handover Request Acknowledge message caninclude the same condition or a reconfigured condition that isnegotiated between the source base station and the candidate basestation.

At block 1106, the source base station transmits (e.g., in any one ofevents 312, 412, 512) the conditional handover configuration andcondition to a user device (e.g., UE 102). The conditional handoverconfiguration and condition can be included in an RRC container or in anRRC reconfiguration message, for example.

At block 1108, the source base station may optionally determine tocancel the conditional handover, and thus send (e.g., in event 864) aHandover Cancel message or Handover Cancel Request message to thecandidate base station.

If the method 1100 includes block 1108, then at block 1110 the sourcebase station may transmit (e.g., in any one of events 865, 866) anotherRRC reconfiguration message to the user device, including an indicationto release the conditional handover configuration and/or the condition.

FIG. 12 is a flow diagram depicting an example method 1200, implementedin a user device (e.g., the UE 102), of performing an immediate handoverduring a conditional handover procedure.

In the method 1200, at block 1202, the user device receives (e.g., inany one of events 312, 412, 512, 612, 614, 616), from a source basestation (e.g., the S-BS 104A), a conditional handover configuration fora conditional handover to a candidate target cell of a candidate basestation (e.g., the C-BS 106A), and a corresponding condition. Theconditional handover configuration and condition can be included in anRRC container message or in a RRC reconfiguration message, for example.

At block 1204, prior to the user device determining that the conditionfor handing over to the candidate target cell of the candidate basestation is satisfied, the user device receives (e.g., in event 613) fromthe source base station a handover command message (e.g., an RRCreconfiguration message) for immediate handover to a target base station(e.g., the T-BS 104B). The handover command message configures a targetcell of the target base station for the user device.

At block 1206, the user device optionally releases (e.g., in event 631)the conditional handover configuration in response to the handovercommand message. Otherwise, the user device may unnecessarily be storingand/or accounting for the conditional handover configuration and thecorresponding condition received at block 1202 from the source basestation.

At block 1208, regardless of whether the user device releases theconditional handover configuration at block 1206, the user devicetransmits (e.g., in event 622) a Handover Complete message (e.g., an RRCreconfiguration complete message) to the target base station in responseto the handover command message, thereby performing an immediatehandover to the target base station. As such, the target base stationbecomes the source base station for the user device, and the user devicestarts communicating data with the source base station.

FIG. 13 is a flow diagram depicting an example method 1300, implementedin a user device (e.g., the UE 102), of managing conditional handoverconfigurations when handing over to a particular candidate base station(e.g., C-BS 106A).

In the method 1300, at block 1302, the user device receives (e.g., inevents 750A/752A/754A, 750B/752B/754B) from a source base station (e.g.,the S-BS 104A) at least two conditional handover configurations (e.g., afirst conditional handover configuration and a second conditionalhandover configuration), and the corresponding conditions, forconditional handover to at least two respective candidate base stations,including a first candidate base station (e.g., the C-BS 106A) and asecond candidate base station (e.g., the C-BS 104B).

At block 1304, the user device determines (e.g., in event 760) that acondition corresponding to the first conditional handover configurationis satisfied.

At block 1306, in response to the determination at block 1304, the userdevice hands over to the first candidate base station using the firstconditional handover configuration. As such, the first candidate basestation becomes the source base station for the user device, and theuser device starts communicating data with the source base station.

At block 1308, in response to the determination at block 1304 or thehandover at block 1306, the user device optionally releases (e.g., inevent 731B) any other conditional handover configurations (e.g., thesecond conditional handover configuration) with which the user devicewas configured at the time of the determination or handover. Otherwise,the user device may unnecessarily be storing and/or accounting for thesecond conditional handover configuration and the correspondingcondition received at block 1302 from the source base station.

At block 1310, the user device may optionally determine (e.g., in event731A) that the first conditional handover configuration is no longervalid because the first conditional handover configuration has alreadybeen used at block 1306. In this way, if the user device later ondetects a failure (e.g., radio link failure) on a communication linkwith the first candidate base station, and responds by selecting back tothe first candidate target cell associated to the first conditionalhandover configuration, the user device will not hand over to the firstcandidate base station using the first conditional handoverconfiguration.

FIG. 14 is a flow diagram depicting an example method 1400, implementedin a source base station (e.g., the S-BS 104A), of selectivelyperforming an immediate handover preparation procedure or a conditionalhandover preparation procedure for a user device (e.g., the UE 102).

In the method 1400, at block 1402, the source base station receivesmeasurement result(s) of one or more cells from the user device. Themeasurement result(s) may include signal-to-noise ratio,signal-to-noise-plus-interference ratio, and/or any other suitablemeasurement indicative of channel quality and/or conditions.

At block 1404, the source base station determines (i.e., checks ordetects) whether a threshold for immediate handover preparation is metaccording to the measurement result(s) received at block 1402 from theuser device. As used herein, a “threshold” can be a singlethreshold/value or a set of thresholds/values.

If the measurement result(s) of a cell to which a target base station(e.g., base station 106A or 104B) belongs satisfy the threshold at block1404, the source base station at block 1406 performs an immediatehandover preparation procedure for the user device, to hand over to thetarget base station (e.g., as described above in connection with FIG.6).

At block 1408, as a result of the immediate handover preparationprocedure performed at block 1406, the source base station transmits(e.g., in event 613) a handover command message (e.g., an RRCreconfiguration message including a ReconfigurationWithSync IE) to theuser device. The handover command message configures a primary cell(PCell) of the target base station for the user device. Upon receivingthe handover command message, the user device hands over to the targetbase station via the PCell by performing a random access procedure withthe target base station on the PCell.

If the measurement result(s) of the cell to which the target basestation belongs instead do not satisfy the threshold at block 1404, thesource base station at block 1410 determines whether a threshold forconditional handover preparation is met according to the measurementresult(s) received from the user device at block 1402.

If the threshold for the conditional handover preparation is met, atblock 1412 the source base station performs a conditional handoverpreparation procedure with a candidate base station to which the cell(s)belong to obtain one or more conditional handover configurations (e.g.,as described above in connection with FIGS. 3-5 and 7-10).

At block 1414, the source base station transmits (e.g., in any one ofevents 312, 412, 512) conditional handover configuration(s) to the userdevice as a result of the conditional handover preparation procedure. Insome implementations, the S-BS 104A may also transmit the condition(s)corresponding to the conditional handover configuration(s). Theconditional handover configuration(s) and condition(s) can be includedin an RRC container or in an RRC reconfiguration message, for example.

If the condition is not met at block 1412, the method 1400 may thenproceed to a new iteration starting at block 1402, for example.

FIG. 15 is a flow diagram depicting an example method 1500, implementedin a source base station (e.g., the S-BS 104A), for reconfiguring orreleasing a conditional handover configuration stored at a user device(e.g., the UE 102).

In the method 1500, at block 1502, the source base station receivesmeasurement result(s) of one or more cells from the user device. Block1502 may be similar to block 1402, for example.

If the source base station determines at block 1504 that the measurementresult(s) are associated to a candidate base station (e.g., the C-BS106A) for which the source base station had configured the user devicefor conditional handover (e.g., in any one of events 350, 452, or 554),the S-BS 104A determines (i.e., checks or detects) at block 1506 whethera threshold for reconfiguring or releasing the conditional handoverconfiguration stored at the user device is met, using the measurementresult(s) received at block 1502 from the user device.

If the measurement result(s) satisfies the threshold, the source basestation performs (e.g., in event 1061) at block 1508 a conditionalhandover release or reconfiguration procedure with the candidate basestation (e.g., in any one of events 864, 1063).

If the measurement result(s) are not determined to be associated to thecandidate base station at block 1504, or the measurement result(s) donot satisfy the threshold at block 1506, the source base station atblock 1510 does not perform the conditional handover release orreconfiguration procedure. The method 1500 may then proceed to a newiteration starting at block 1502, for example.

FIG. 16 is a flow diagram depicting an example method 1600, implementedin a user device (e.g., the UE 102), for performing an immediate orconditional handover when a communication failure occurs.

In the method 1600, at block 1602, the user device receives, from asource base station (e.g., S-BS 104A) and while operating in a sourcecell covered by the source base station, one or more sets of conditionsto be satisfied before the user device performs a handover to one ormore respective candidate target cells.

At block 1604, the user device detects a communication failure (e.g., aradio link failure or other communication failure) between the userdevice and the source base station.

At block 1606, in response to detecting the communication failure, theuser device selects a new cell.

At block 1608, the user device performs, based on whether the new cellis one of the one or more respective candidate target cells, either anRRC reestablishment procedure to the new cell or a conditional handoverto the new cell. Particularly, if the user device determines that thenew cell is a candidate target cell (i.e., a cell associated to aconditional handover configuration), the user device performs (e.g., inany one of events 360, 460, 560, 760) conditional handover to the newcell using the conditional handover configuration if a conditioncorresponding to the conditional handover configuration is met (andoptionally later determines, e.g., in event 731A, that the conditionalhandover configuration is no longer valid). In some implementations, ifthe user device fails a random access procedure when performing theconditional handover, the user device can keep the invalid conditionalhandover configuration (and any remaining conditional handoverconfiguration(s)), or alternatively release the invalid conditionalhandover configuration.

If the user device determines that the new cell is not a candidatetarget cell (i.e., not a cell associated with an immediate handoverconfiguration), the user device performs (e.g., in event 620) the RRCreestablishment procedure with the new cell, and optionally releases(e.g., in any one of events 631, 731B) one or more conditional handoverconfigurations corresponding to the one or more sets of conditionsreceived from the source base station at block 1602. In someimplementations, if the user device fails a random access procedure whenperforming the RRC reestablishment procedure, the user device can keepthe conditional handover configuration(s). In this way, if the userdevice subsequently selects another cell that is associated to anotherthe conditional handover configuration(s), the user device can performconditional handover to that cell according to the correspondingconditional handover configuration. Alternatively, the user devicereleases the conditional handover configuration(s) when failing therandom access procedure.

FIG. 17 is a flow diagram depicting an example method 1700, implementedin a user device (e.g., the UE 102), for managing conditional handoverconfigurations when performing conditional handover.

In the method 1700, at block 1702, the user device receives (e.g., inevent 750A/752A/754A), from a source base station (e.g., the S-BS 104A)and while the user device is operating in a source cell covered by thesource base station, a first conditional handover configurationproviding information for user device operation within a first candidatetarget cell covered by a first candidate base station (e.g., the C-BS106A).

At block 1704, the user device monitors whether a first set of one ormore conditions associated with a conditional handover to the firstcandidate target cell is satisfied.

At block 1706, the user device receives (e.g., in event 750B/752B/754B),from the source base station and while the user device is operating inthe source cell, a second conditional handover configuration providinginformation for user device operation within a second candidate targetcell covered by a second candidate base station (e.g., the C-BS 104B).

At block 1708, the user device monitors whether a second set of one ormore conditions associated with a conditional handover to the secondcandidate target cell is satisfied.

At block 1710, the user device determines (e.g., in event 760) that thefirst set of conditions is satisfied, and in response, at block 1712,performs a handover to the first candidate target cell using the firstconditional handover configuration.

In contrast to block 1308 of FIG. 13 in which the user device releasesthe second conditional handover configuration in response to handingover to the first candidate base station, the user device may keep thesecond conditional handover configuration. As such, at block 1714, afterperforming the handover, the user device can continue to monitor whetherthe second set of conditions is satisfied. If the second set ofconditions is satisfied, the user device can handover to the secondcandidate base station using the kept second conditional handoverconfiguration.

FIG. 18 is a flow diagram depicting an example method 1800, implementedin a source base station (e.g., the S-BS 104A), for providing conditionsfor a conditional handover to a base station associated with a candidatetarget cell (e.g., the C-BS 106A).

In the method 1800, at block 1802, the source base station determines(e.g., in event 404), while a user device (e.g., the UE 102) isoperating in a source cell covered by the source base station, toconfigure a conditional handover for the user device.

At block 1804, the source base station identifies at least a firstcandidate target cell for the conditional handover.

At block 1806, the source base station transmits (e.g., in event 406),to the first candidate base station that covers the first candidatetarget cell, a first set of one or more conditions to be satisfiedbefore the user device performs a handover to the first candidate targetcell.

FIG. 19 is a flow diagram depicting an example method 1900, implementedin a candidate base station (e.g., the C-BS 106A), for facilitation thesharing and/or negotiation of conditions between a source base station(e.g., the S-BS 104A) and a candidate target base station (e.g., theC-BS 106A).

In the method 1900, at block 1902, the candidate base station receives(e.g., in event 406), from the source base station covering a sourcecell in which a user device (e.g., the UE 102) is operating, (1) arequest for a conditional handover to the candidate base station and (2)a first set of one or more conditions to be satisfied before the userdevice performs a handover to the candidate base station.

At block 1904, the candidate base station determines, based on the firstset of conditions, a second set of one or more conditions. For example,if the candidate base station determines that the condition(s) receivedfrom the source base station include a low threshold such that the userdevice would too easily determine that the condition(s) are satisfied,the candidate base station can determine a second set of condition(s)that includes a higher threshold.

At block 1906, the candidate base station transmits (e.g., in event408), to the source base station, a message including the second set ofconditions. The source base station can either accept or reject thesecond set of conditions.

FIG. 20 is a flow diagram depicting an example method 2000, implementedin a RAN (e.g., a RAN including the S-BS 104A and C-BS 106A), formodifying a conditional handover configuration that was previously sentto the UE.

In the method 2000, at block 2002, the RAN transmits (e.g., in procedure1050/1052/1054), to a user device (e.g., the UE 102), a conditionalhandover configuration providing information for user device operationwithin a candidate target cell covered by a candidate base station ofthe RAN (e.g., cell 126A of C-BS 106A).

At block 2004, after transmitting the conditional handover configurationto the user device, the RAN determines (e.g., in any one of events 1061,1062) to modify or replace the conditional handover configuration.

At block 2006, the RAN transmits (e.g., in event 1065) a modifiedconditional handover configuration to the user device.

The following additional considerations can apply to the foregoingdiscussion.

In some implementations, the source base station (e.g., S-BS 104A) mayset different or the same condition(s) for respective candidate basestation(s) (e.g., C-BS 106A, C-BS 104B), or for respective candidatetarget cells of the respective candidate base station(s). In oneimplementation, the source base station may set the condition(s)differently based on the coverages of different candidate base stationsor candidate target cells. For example, the S-BS 104A may set a firstthreshold in a first condition for a first C-BS 106A or a firstcandidate target cell, and set a second threshold (e.g., greater thanthe first threshold) in a second condition for a second C-BS 104B or asecond candidate target cell if coverage of the first C-BS 106A or thefirst candidate target cell is larger/smaller than coverage of thesecond C-BS 104B or the second candidate target cell.

In another implementation, the source base station may set thecondition(s) differently based on different carrier frequencies on whichthe candidate base stations or the candidate target cells operate, ormay set the same condition(s) if the candidate base stations or thecandidate target cells operate on the same frequencies. For example, theS-BS 104A may set a first threshold in a first condition for a firstC-BS106A or a first candidate target cell, and set a second threshold(e.g., greater than the first threshold) in a second condition for asecond C-B S 104B or a second candidate target cell if a downlinkcarrier frequency of the first C-BS or the first candidate target cellis lower/higher than a downlink carrier frequency of the second C-BS orthe second candidate target cell.

In some implementations, a candidate base station (e.g., C-BS 106A) maystart a first timer in response to the conditional handover preparationfor the UE (e.g., UE 102). If the first timer expires, the C-BS 106Areleases a conditional handover configuration for the UE 102. If theC-BS 106A connects to the UE 102 (e.g., in a random access procedure asdescribed above), the C-BS 106A stops the first timer for the UE 102. Atarget base station (e.g., T-BS 104B) may start a second timer inresponse to the immediate handover preparation for a second UE (e.g.,another UE similar to the UE 102) that has a shorter value than thefirst timer. If the second timer expires, the T-BS 104B releases animmediate handover configuration for the second UE. If the T-BS 106Aconnects to the second UE in a random access procedure, the T-BS 104Bstops the second timer.

In some implementations, if a DU of the candidate base station (e.g.,C-BS 106A) identifies the UE (e.g., UE 102) in the random accessprocedure, the DU may send a RAN node interface message to a CU of thecandidate base station. In some implementations, the RAN node interfacemessage is a F1 AP message defined in 3GPP Technical Specification38.473. In one implementation, the F1 AP message indicates that the UEis connected. In other implementations, the F1 AP message includes theidentity of the UE, to enable the CU to identify the UE or determinethat the UE is connected. In another implementation, the interfacemessage is a DL DATA DELIVERY STATUS frame as defined in NR user planeprotocol. As soon as the DU detects the successful random accessprocedure (RACH) access by the UE for the corresponding data radiobearer(s), the DU sends the DL DATA DELIVERY STATUS frame to the CU.

In some implementations, the RRC reconfiguration messages discussedabove can be RRCConnectionReconfiguration messages, and the RRCreconfiguration complete messages can beRRCConnectionReconfigurationComplete messages. In other implementations,the RRC reconfiguration messages can be RRCReconfiguration messages andthe RRC reconfiguration complete messages can beRRCReconfigurationComplete messages.

A user device in which the techniques of this disclosure can beimplemented (e.g., the UE 102) can be any suitable device capable ofwireless communications such as a smartphone, a tablet computer, alaptop computer, a mobile gaming console, a point-of-sale (POS)terminal, a health monitoring device, a drone, a camera, amedia-streaming dongle or another personal media device, a wearabledevice such as a smartwatch, a wireless hotspot, a femtocell, or abroadband router. Further, the user device in some cases may be embeddedin an electronic system such as the head unit of a vehicle or anadvanced driver assistance system (ADAS). Still further, the user devicecan operate as an internet-of-things (IoT) device or a mobile-internetdevice (MID). Depending on the type, the user device can include one ormore general-purpose processors, a computer-readable memory, a userinterface, one or more network interfaces, one or more sensors, etc.

Certain embodiments are described in this disclosure as including logicor a number of components or modules. Modules may can be softwaremodules (e.g., code, or machine-readable instructions stored onnon-transitory machine-readable medium) or hardware modules. A hardwaremodule is a tangible unit capable of performing certain operations andmay be configured or arranged in a certain manner. A hardware module cancomprise dedicated circuitry or logic that is permanently configured(e.g., as a special-purpose processor, such as a field programmable gatearray (FPGA) or an application-specific integrated circuit (ASIC), adigital signal processor (DSP), etc.) to perform certain operations. Ahardware module may also comprise programmable logic or circuitry (e.g.,as encompassed within a general-purpose processor or other programmableprocessor) that is temporarily configured by software to perform certainoperations. The decision to implement a hardware module in dedicated andpermanently configured circuitry, or in temporarily configured circuitry(e.g., configured by software) may be driven by cost and timeconsiderations.

When implemented in software, the techniques can be provided as part ofthe operating system, a library used by multiple applications, aparticular software application, etc. The software can be executed byone or more general-purpose processors or one or more special-purposeprocessors.

The following list of aspects reflects a variety of the embodimentsexplicitly contemplated by the present disclosure.

Aspect 1. A method, in a user device, comprising: receiving, byprocessing hardware of the user device and from a source base stationwhile the user device is operating in a source cell of the source basestation, a conditional handover configuration providing information foruser device operation within a candidate target cell of a candidate basestation, and a corresponding condition for handing over to the candidatetarget cell of the candidate base station; prior to determining that thecondition is satisfied, receiving, by the processing hardware and fromthe source base station, a handover command message for immediatehandover to a target base station; and after receiving the handovercommand message, releasing, by the processing hardware, the conditionalhandover configuration, and performing an immediate handover to thetarget base station.

Aspect 2. The method of aspect 1, wherein performing the immediatehandover to the target base station includes performing a random accessprocedure with the target base station, and wherein releasing theconditional handover configuration comprises releasing the conditionalhandover configuration after performing the random access procedure.

Aspect 3. The method of aspect 1, wherein releasing the conditionalhandover configuration comprises releasing the conditional handoverconfiguration in response to the handover command message.

Aspect 4. The method of aspect 3, wherein the handover command messageincludes an indication to release the conditional handoverconfiguration, and wherein releasing the conditional handoverconfiguration in response to receiving the handover command messagecomprises releasing the conditional handover configuration in responseto the indication included in the handover command message.

Aspect 5. The method of any one of aspects 1 through 4, whereinreceiving the conditional handover configuration and the correspondingcondition includes receiving an RRC container message that includes (i)an RRC reconfiguration that includes the conditional handoverconfiguration and (ii) the corresponding condition.

Aspect 6. A method, in a user device, comprising: receiving, from asource base station and while the user device is operating in a sourcecell of the source base station, a first conditional handoverconfiguration providing information for user device operation within afirst candidate target cell of a first candidate base station;monitoring, by processing hardware of the user device, whether a firstcondition associated with a conditional handover to the first candidatetarget cell is satisfied; receiving, from the source base station andwhile the user device is operating in the source cell, a secondconditional handover configuration providing information for user deviceoperation within a second candidate target cell covered by a secondcandidate base station; monitoring, by the processing hardware, whethera second condition associated with a conditional handover to the secondcandidate target cell is satisfied; determining, by the processinghardware and while monitoring whether the first and second conditionsare satisfied, that the first condition is satisfied; and in response todetermining that the first condition is satisfied, performing a handoverto the first candidate target cell, and releasing, by the processinghardware, the second conditional handover configuration.

Aspect 7. The method of aspect 6, further comprising: after performingthe handover to the first candidate target cell, determining, by theprocessing hardware, that the first conditional handover configurationis no longer valid.

Aspect 8. The method of aspect 6, wherein releasing the secondconditional handover configuration further comprises releasing allconditional handover configurations, other than the first conditionalhandover configuration, with which the user device was configured.

Aspect 9. A user device comprising hardware and configured to performthe method of any one of aspects 1 through 8.

Aspect 10. A method, in a base station, comprising: sending, byprocessing hardware of the base station and to a user device while theuser device is operating in a source cell of the base station, aconditional handover configuration providing information for user deviceoperation within a candidate target cell of a candidate base station,and a corresponding condition for handing over to the candidate targetcell of the candidate base station; and causing, by the processinghardware, the user device to release the conditional handoverconfiguration and perform an immediate handover to a target base stationprior to the user device determining that the condition is satisfied, atleast by sending, to the user device, a handover command message forimmediate handover to the target base station.

Aspect 11. The method of aspect 10, wherein causing the user device toperform the immediate handover to the target base station includescausing the user device to perform a random access procedure with thetarget base station, and wherein causing the user device to release theconditional handover configuration comprises causing the user device torelease the conditional handover configuration after performing therandom access procedure.

Aspect 12. The method of aspect 10, wherein causing the user device torelease the conditional handover configuration comprises causing theuser device to release the conditional handover configuration inresponse to the handover command message.

Aspect 13. The method of aspect 12, wherein the handover command messageincludes an indication to release the conditional handoverconfiguration, and wherein causing the user device to release theconditional handover configuration in response to the handover commandmessage comprises causing the user device to release the conditionalhandover configuration in response to the indication included in thehandover command message.

Aspect 14. The method of any one of aspects 10 through 13, whereinsending the conditional handover configuration and the correspondingcondition includes sending an RRC container message that includes (i) anRRC reconfiguration that includes the conditional handover configurationand (ii) the corresponding condition.

Aspect 15. A method, in a base station, comprising: sending, byprocessing hardware of the base station and to a user device while theuser device is operating in a source cell of the base station, a firstconditional handover configuration providing information for user deviceoperation within a first candidate target cell of a first candidate basestation when a first condition associated with a conditional handover tothe first candidate target cell is satisfied; sending, by the processinghardware and to the user device while the user device is operating inthe source cell, a second conditional handover configuration providinginformation for user device operation within a second candidate targetcell covered by a second candidate base station when a second conditionassociated with a conditional handover to the second candidate targetcell is satisfied; and causing, by the processing hardware, the userdevice to release the second conditional handover configuration andperform a handover to the first candidate target cell after the userdevice determines that the first condition is satisfied.

Aspect 16. The method of aspect 15, further comprising: causing, by theprocessing hardware, the user device to determine that the firstconditional handover configuration is no longer valid.

Aspect 17. The method of aspect 15, wherein causing the user device torelease the second conditional handover configuration further comprisescausing the user device to release all conditional handoverconfigurations, other than the first conditional handover configuration,with which the user device was configured.

Aspect 18. A base station comprising hardware and configured to performthe method of any one of aspects 10 through 17.

What is claimed is:
 1. A method, in a user device, comprising:receiving, by processing hardware of the user device and from a sourcebase station while the user device is operating in a source cell of thesource base station, a conditional handover configuration providinginformation for user device operation within a candidate target cell ofa candidate base station, and a corresponding condition for handing overto the candidate target cell of the candidate base station; prior todetermining that the condition is satisfied, receiving, by theprocessing hardware and from the source base station, a handover commandmessage for immediate handover to a target base station; and afterreceiving the handover command message, releasing, by the processinghardware, the conditional handover configuration, and performing animmediate handover to the target base station.
 2. The method of claim 1,wherein performing the immediate handover to the target base stationincludes performing a random access procedure with the target basestation, and wherein releasing the conditional handover configurationcomprises releasing the conditional handover configuration afterperforming the random access procedure.
 3. The method of claim 1,wherein releasing the conditional handover configuration comprisesreleasing the conditional handover configuration in response to thehandover command message.
 4. The method of claim 3, wherein the handovercommand message includes an indication to release the conditionalhandover configuration, and wherein releasing the conditional handoverconfiguration in response to receiving the handover command messagecomprises releasing the conditional handover configuration in responseto the indication included in the handover command message.
 5. Themethod of claim 1, wherein receiving the conditional handoverconfiguration and the corresponding condition includes receiving an RRCcontainer message that includes (i) an RRC reconfiguration that includesthe conditional handover configuration and (ii) the correspondingcondition.
 6. A method, in a user device, comprising: receiving, from asource base station and while the user device is operating in a sourcecell of the source base station, a first conditional handoverconfiguration providing information for user device operation within afirst candidate target cell of a first candidate base station;monitoring, by processing hardware of the user device, whether a firstcondition associated with a conditional handover to the first candidatetarget cell is satisfied; receiving, from the source base station andwhile the user device is operating in the source cell, a secondconditional handover configuration providing information for user deviceoperation within a second candidate target cell covered by a secondcandidate base station; monitoring, by the processing hardware, whethera second condition associated with a conditional handover to the secondcandidate target cell is satisfied; determining, by the processinghardware and while monitoring whether the first and second conditionsare satisfied, that the first condition is satisfied; and in response todetermining that the first condition is satisfied, performing a handoverto the first candidate target cell, and releasing, by the processinghardware, the second conditional handover configuration.
 7. The methodof claim 6, further comprising: after performing the handover to thefirst candidate target cell, determining, by the processing hardware,that the first conditional handover configuration is no longer valid. 8.The method of claim 6, wherein releasing the second conditional handoverconfiguration further comprises releasing all conditional handoverconfigurations, other than the first conditional handover configuration,with which the user device was configured.
 9. A method, in a basestation, comprising: sending, by processing hardware of the base stationand to a user device while the user device is operating in a source cellof the base station, a conditional handover configuration providinginformation for user device operation within a candidate target cell ofa candidate base station, and a corresponding condition for handing overto the candidate target cell of the candidate base station; and causing,by the processing hardware, the user device to release the conditionalhandover configuration and perform an immediate handover to a targetbase station prior to the user device determining that the condition issatisfied, at least by sending, to the user device, a handover commandmessage for immediate handover to the target base station.
 10. Themethod of claim 9, wherein causing the user device to perform theimmediate handover to the target base station includes causing the userdevice to perform a random access procedure with the target basestation, and wherein causing the user device to release the conditionalhandover configuration comprises causing the user device to release theconditional handover configuration after performing the random accessprocedure.
 11. The method of claim 9, wherein causing the user device torelease the conditional handover configuration comprises causing theuser device to release the conditional handover configuration inresponse to the handover command message.
 12. The method of claim 11,wherein the handover command message includes an indication to releasethe conditional handover configuration, and wherein causing the userdevice to release the conditional handover configuration in response tothe handover command message comprises causing the user device torelease the conditional handover configuration in response to theindication included in the handover command message.
 13. The method ofclaim 9, wherein sending the conditional handover configuration and thecorresponding condition includes sending an RRC container message thatincludes (i) an RRC reconfiguration that includes the conditionalhandover configuration and (ii) the corresponding condition.